Libyan authorities have made four arrests in the investigation into the attack on the U.S. Consulate in Benghazi in which the U.S. ambassador and three embassy staff were killed, the deputy interior minister said on Thursday.

"Four men are in custody and we are interrogating them because they are suspected of helping instigate the events at the U.S. Consulate," Wanis Sharif told Reuters.

He gave no more details.

The United States and Libya has agreed to cooperate to find out who was responsible for the deadly attack on the U.S. Consulate in Benghazi in which the ambassador to the North African state and three other Americans died.

President Barack Obama and Libyan President Mohamed Magarief spoke on Wednesday evening and decided "to work closely over the course of this investigation," the White House said in a statement.

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Magarief "expressed appreciation for the cooperation we have received from the Libyan government and people in responding to this outrageous attack, and said that the Libyan government must continue to work with us to assure the security of our personnel going forward," the White House statement said.

"The President made it clear that we must work together to do whatever is necessary to identify the perpetrators of this attack and bring them to justice," it added.

U.S. and Libyan officials, independent analysts and postings on Islamist websites from known militant activists suggested that the attack — which officials had previously suggested was retaliation for release of a movie critical of the Muslim Prophet Muhammad — may have been a pre-planned, orchestrated assault.

Ambassador Christopher Stevens, Sean Smith — a Foreign Service information management officer — and two other Americans, who have not yet been formally identified, were killed.

Destroyers sent to Libya coast
A U.S. official told Reuters that the U.S. military was moving two destroyers toward the Libyan coast, giving the Obama administration flexibility for any future action against Libyan targets.

Timeline: Political fallout from the attack on diplomats in Libya

The military is also dispatching a Marine Corps anti-terrorist security team to boost security in Libya, and Washington has ordered the evacuation of all U.S. personnel from Benghazi to Tripoli.

An unnamed senior U.S. official told the AFP news agency that U.S. officials suspected the attack on the consulate was a well-planned assault by militants rather than a rampaging mob.

"That's the working hypothesis at the moment," said the official, who spoke on condition of anonymity.

"This was a complex attack," he added. "They seemed to have used this (protest) as an opportunity."

Among the assailants, Libyans identified units of a heavily armed local Islamist group, Ansar al-Sharia, which sympathizes with al-Qaida and derides Libya's U.S.-backed bid for democracy.

Reuters cited U.S. officials as saying that there were reports from the region suggesting that members of al-Qaida's north Africa-based affiliate, known as Al-Qaida in the Islamic Maghreb, may have been involved.

The attack on the Libyan consulate, as it happened

On Wednesday, Obama vowed to catch those responsible for the attack and said he had ordered an increase in security at U.S. diplomatic posts around the globe following the assault.

"The United States condemns in the strongest terms this outrageous and shocking attack," Obama said, while insisting it would not threaten relations with Libya's new government. ... And make no mistake, we will work with the Libyan government to bring to justice the killers who attacked our people."

Doctor tried to save ambassador's life
Ziad Abu Zaid, the duty doctor in the emergency room at Benghazi Medical Center on Tuesday, said Stevens was alive when he arrived at the hospital.

"He came in a state of cardiac arrest. I performed CPR for 45 minutes, but he died of asphyxiation due to smoke inhalation,” he said.

Stevens' body was later returned to U.S. custody at Benghazi airport, a senior U.S. official said. Images of Stevens, purportedly taken after he died, circulated on the Internet. One showed him being carried, with a white shirt pulled up and a cut on his forehead.

Smith died inside the consulate building and the two other Americans died when a squad of U.S. troops sent by helicopter from Tripoli to rescue the diplomats came under mortar attack, said Captain Fathi al-Obeidi, commander of a Libyan special operations unit ordered to meet the Americans.

Obama: Egypt not an ally of US, but not an enemy

Witnesses said the mob at the consulate included tribesmen, militia and other gunmen. Hamam, a 17-year-old who took part in the attack, said Ansar al-Sharia cars arrived at the start of the protest but left once fighting started.

"The protesters were running around the compound just looking for Americans, they just wanted to find an American so they could catch one," he told Reuters. "We started shooting at them, and then some other people also threw hand-made bombs over the fences and started the fires in the buildings."

"There was some Libyan security for the embassy outside but when the hand-made bombs went off they ran off and left," he added.

Hamam said he saw an American die in front of him in the mayhem that ensued. He said the body was covered in ash. 

Reuters contributed to this report.

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BENGHAZI, Libya -- U.S. troops dispatched by helicopter across the Libyan desert to rescue Ambassador Chris Stevens and other diplomats in Benghazi ran into a fierce overnight ambush that left a further two Americans dead, Libyan officials told Reuters.

Accounts of the mayhem at the U.S. consulate, where the ambassador and another diplomat died after a chaotic protest over a film insulting to Islam, remain patchy. But two Libyan officials, including the commander of a security force that escorted the U.S. rescuers, said a later assault on a supposedly safe refuge for the diplomats appeared professionally executed.

Miscommunication that understated the number of American survivors awaiting rescue -- there were 37, nearly four times as many as the Libyan commander expected -- also meant survivors and rescuers found themselves short of transport to escape this second battle, delaying an eventual dawn break for the airport.

Capt. Fathi al-Obeidi, whose special operations unit was ordered by Libya's authorities to meet an eight-man U.S. Marine force at Benghazi airport, said that after his men and the Marines had found the American survivors who had evacuated the blazing consulate, the ostensibly secret location in an isolated villa came under an intense and highly accurate mortar barrage.

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"I really believe that this attack was planned," he said, adding to suggestions by other Libyan officials that at least some of the hostility towards the Americans was the work of experienced combatants. "The accuracy with which the mortars hit us was too good for any regular revolutionaries."

Obeidi's Libya Shield Brigade was formed by civilians during last year's U.S.-backed uprising against Moammar Gadhafi and is now part of the ad hoc government militia forces which the fledgling democratic administration uses to keep order.

Other Libyan officials cited the possible involvement of former soldiers still loyal to Gadhafi's family or Islamist fighters, some of whom have trained and fought in Afghanistan.

U.S. officials have noted it was "complex attack". Several Libyan officials and witnesses said an initial demonstration at the consulate appeared to be largely unarmed, though some elements of an Islamist militia were spotted.

Related: What's known about the consulate attack

At some point, the crowd became incensed, believing they were under attack from within the consulate, many fetched weapons and the consular villa ended up in flames, with most of the Americans fleeing to the safe house after two, including Stevens, had been fatally injured.

'Six mortars' on path to villa
Of the eight American troops who had come from Tripoli, one was killed and two were wounded, Obeidi said. A Libyan deputy interior minister said a second American was also killed in the attack on the safe house. It was not clear if this was a diplomat or one of the consulate's original security detail.

"It began to rain down on us," Obeidi told Reuters, describing the moment the attack began -- just as the Libyan security force was starting up the 10 pickup trucks and sedans they had brought to ferry the Americans to the airport.

"About six mortars fell directly on the path to the villa," he said. "During this firing, one of the Marines whom I had brought with me was wounded and fell to the ground.

"As I was dragging the wounded Marine to safety, some Marines who were located on the roof of the villa as snipers shouted and the rest of the Marines all hit the ground.

"A mortar hit the side of the house. One of the Marines from the roof went flying and fell on top of us."

A senior U.S. diplomat -- not Ambassador Stevens, who Libyan officials said died at a local hospital of the effects of smoke -- urged pushing ahead with the evacuation, Obeidi said.

But he had a transport problem. Having been told to expect 10 Americans and having found 37, Obeidi did not have enough vehicles to break out, despite having one heavy anti-aircraft gun mounted on a pickup truck. 

"I was being bombarded by calls from all over the country by Libyan government officials who wanted me to hurry and get them out," he said. "But I told them that we were in such difficult circumstances and that I needed more men and more cars."

Eventually dozens more vehicles were dispatched from pro-government militia brigades and, with the sun rising, the convoy headed back to the airport where an aircraft flew a first group of U.S. personnel out to the Libyan capital.

Libyan Deputy Interior Minister Wanis al-Sharif said Stevens and another diplomat died in the first series of incidents around the consulate, while the other two Americans died during the attempt to evacuate from the safe house to the airport.

"(The ambassador) died as a result of suffocation by the fumes of the fire inside the embassy and one was also killed by gunfire before around 37 people were moved to a place we thought was safe," Sharif told Reuters in Benghazi.

Speaking of the rescue mission, he said: "A team of commandos arrived by air and went to a farm which we thought was a secret location. Once they got there, they came under heavy fire from heavy machine guns, rocket-propelled grenades and automatic rifles, which resulted in the death of two others."

He estimated that a dozen or more Americans were hurt.

More world stories from NBC News:

• US won't rule out Islamist link in killing of US ambassador to Libya
• US Ambassador Chris Stevens was 'courageous and exemplary,' Obama says
• Romney slams Obama over attacks on US officials in Libya, Egypt
• Report: Maker of Muhammed film goes into hiding
• Despite dark past, young Israelis seek new lives in German capital
• No Obama-Netanyahu meeting as rift over Iran widens

Follow World News from NBCNews.com on Twitter and Facebook

 

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