SEOUL -- South Korean police were investigating a hacking attack on an Internet provider that brought down the servers of three broadcasters and two major banks on Wednesday, and the army raised its alert level due to concerns of North Korean involvement.

The network provided by LG UPlus Corp. showed a page that said it had been hacked by a group calling itself the "Whois Team," an unknown group. It featured three skulls and a warning that this was the beginning of "Our Movement."

Servers at television networks YTN, MBC and KBS were affected as well as Shinhan Bank and NongHyup Bank, both major financial institutions, police and government officials said.

"We sent down teams to all affected sites. We are now assessing the situation. This incident is pretty massive, and it will take a few days to collect evidence," a police official said.

Police and government officials declined to speculate on whether North Korea, which has threatened to attack both South Korea and the United States after it was hit with United Nations sanctions for its February nuclear test, was behind the cyberattack.

North Korea has in the past staged cyberattacks on the world's most wired country, targeting conservative newspapers, banks and government institutions.

South Korea's military said it was not affected but raised its state of readiness in response.

None of South Korea's oil refineries, power stations, ports or airports was affected.

The biggest attack by Pyongyang was a 10-day denial of service attack in 2011 that antivirus firm McAfee, part of Intel Corp, dubbed "Ten Days of Rain" and which it said was a bid to probe the South's computer defenses in the event of a real conflict.

Shinhan Bank, one of the financial institutions affected, said its servers were back up by 4 p.m. local time (3 a.m. ET).

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This story was originally published on Wed Mar 20, 2013 2:49 AM EDT