Monsignor Oscar Sanchez of Mexico, who was at the Vatican for the announcement, was quoted by The Associated Press as saying that the cardinals “remained shocked and were looking at each other.”

President Barack Obama said in a statement that he and first lady Michelle Obama “warmly remember our meeting with the Holy Father in 2009, and I have appreciated our work together over these last four years.”

Canon law says that the pope may relinquish his office provided that the decision is “made freely and properly manifested” — language to which Benedict appeared to allude in his statement.

Because there is no one in the church higher than the pope to accept a resignation, the renouncement is technically an abdication.

The last pope universally recognized to have abdicated was Celestine V, who was elected in July 1294 and gave up the job five months later after feeling that he was being manipulated by the King of Sicily and Naples. He was declared a saint in 1313.

During a period of division known as the Great Western Schism, from 1378 to 1415, there were three rival claimants to the papacy. The legitimate pope, Gregory XII, abdicated to make way for an undisputed pope.

Benedict’s abdication clears the way for the College of Cardinals to gather at the Vatican to elect a successor, a process in which the United States is expected to have unprecedented sway.

The U.S. will have 11 votes, almost 10 percent of the electorate and the second-largest voting bloc behind Italy, which will have 28 votes. Germany, the home country of the current pope, will have six.

It appears highly unlikely that an American will be elected Benedict’s successor. Dolan, president of the U.S. Conference of Catholic Bishops, is considered a longshot for the job.

Among the cardinals mentioned as possible successors are Angelo Scola of Italy, Peter Turkson of Ghana, Marc Ouellet of Canada and Francis Arinze of Nigeria and Christoph Schoenborn of Austria.

Cardinal Donald Wuerl, the archbishop of Washington, told reporters that the cardinals would be looking for an “articulate voice” for the church and would keep in mind Benedict’s tradition.

“He has called all of us to focus on the spiritual mission of the church, proclaim the gospel and once again begin this personal relationship all of us are capable of having with God back to the foreground,” he told reporters at St. Matthew’s Cathedral.

Benedict, then Cardinal Joseph Ratzinger, was elected April 19, 2005. He was the 265th pope and the successor to John Paul II, who had served since 1978 and was wildly popular among the faithful.

Born in 1927, he had been conscripted into the Hitler Youth during World War II, but he never joined the Nazi Party, and his family opposed the regime of Adolf Hitler, Reuters reported.

Ratzinger, before being elevated to pope, headed the Vatican’s Congregation for the Doctrine of the Faith, which oversees church doctrine. His strict approach to theology earned him the nickname “God’s Rottweiler.”

He sought to rekindle the faith of Catholics and bring them closer to the teachings of the church. He worried that too many had strayed, and said in 2005 that the parts of the world suffered from “a strange forgetfulness of God.”

During Benedict’s papacy, thousands of people came forward to report that priests had raped or molested them as children and that bishops had covered it up.

It was Benedict’s old office that dealt with abuse cases, yet Benedict never admitted failure himself or of the Vatican, and never punished bishops who ignored or covered up the abuse.

“He could go around and minister to victims, which he did, and I think that was a brave and profound thing to do, but he couldn’t change the definitive elements of the Catholic Church that enable abuse,” said Michael D’Antonio, author of “Mortal Sins: Sex, Crime, and the Era of Catholic Scandal.”

“He would have had to pick up the church and drag it into the 21st century, but you know he could have,” he said. “He might have died trying, the stress of that might have been even more profound, he would have faced tremendous intrigue and opposition but I suspect that instead he may go down in history as a caretaker, an interpersonally kind pastor who made no mark when he had the chance to.”

Bill Donahue, president of the Catholic League, said that Benedict had tackled the abuse problem much more aggressively than John Paul II, who he said had let the issue languish.

“Nobody clearly did more to counter this problem in the Catholic Church” than Benedict, Donahue said. “I think history will treat him very well in terms of dealing with the problem.”

Benedict continued the outreach to Jews of his predecessor, John Paul II, and was the second pope to enter a synagogue. His relationship with Muslims, however, was much more complex.

He generated outrage among Muslims when, in 2006, he gave a speech in Germany and quoted a Byzantine emperor who had characterized some of the Prophet Muhammad’s teachings “as evil and inhuman.”

Benedict also stirred an uproar in 2009 when, en route to Africa and discussing the AIDS epidemic with reporters, he said that the distribution of condoms “increases the problem” rather than preventing the spread of the disease.

A year later, in an interview, he said that a male prostitute who used a condom to avoid passing HIV to his partner might be taking a step toward more responsible sexuality.

James Salt, executive director of Catholics United, which claims 40,000 members and wants the church to focus more on social justice and poverty, praised the abdication as a “sign of humility from the aging Holy Father” and encouraged the church to reflect on the “challenges of this papacy.”

He suggested that the church open itself to a pope from Latin America or Africa.

Slideshow: The life of Pope Benedict XVI

Joseph Ratzinger became Pope Benedict XVI in 2005. Look back at his life from childhood through his papacy.

Launch slideshow

NBC News staff writer Miranda Leitsinger, Reuters and The Associated Press contributed to this report.

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