Police on Friday investigated the motives of the anti-U.S. activist they say slashed the U.S. ambassador to South Korea as questions turned to whether security was neglected.

The attack Thursday on Mark Lippert, which prompted rival North Korea to gloat about "knife slashes of justice," left deep gashes and damaged tendons and nerves. But it also raised safety worries in a city with a reputation as a relatively low-risk diplomatic posting, despite regular threats of war from Pyongyang.

While an extreme example, the attack is the latest act of political violence in a deeply divided country where some protesters portray their causes as matters of life and death.

Lippert, 42, was recovering well but complaining of pain in the wound on his left wrist and a finger where doctors repaired nerve damage, Severance Hospital official Yoon Do-Heum said in a televised briefing. Doctors will remove the 80 stiches on Lippert's face on Monday or Tuesday and expect him to be out of the hospital by Tuesday or Wednesday. Hospital officials say he may experience sensory problems in his left hand for several months.

Police, meanwhile, searched the offices of the suspect, Kim Ki-jong, 55, for documents and computer files as they investigated how the attack was planned and whether others were involved. Police plan to soon request a warrant for Kim's formal arrest, and potential charges include attempted murder, assaulting a foreign envoy, obstruction and violating a controversial South Korean law that bars praise or assistance of North Korea, Jongno district police chief Yun Myung-sung told reporters.

Police are also looking into Kim's past travels to North Korea — seven times between 1999 and 2007 — during a previous era of inter-Korean cooperation, when Seoul was ruled by a liberal government.

Kim, who has a long history of anti-U.S. and violent protests, said he acted alone in the attack on Lippert. He told police it was meant as a protest of annual U.S.-South Korean military drills that started Monday — exercises that the North has long maintained are preparations for an invasion. Kim said the drills, which Seoul and Washington say are purely defensive, ruined efforts for reconciliation between the Koreas, according to police officials.

Security for Lippert was another focus.

U.S. ambassadors have security details, but their size largely depends on the threat level of the post. It's not clear how many guards Lippert had at the downtown venue where he was attacked, but since Seoul is seen as fairly safe, the number of guards would have been fewer than for the American ambassadors in most of the Mideast.

The U.S. Embassy, citing security reasons, only informed South Korean police what Lippert was doing a little before Thursday's event, and the U.S. Embassy provided its own security detail for the ambassador, according to a Seoul police officer who spoke on condition of anonymity, citing office rules. The U.S. Embassy in Seoul didn't immediately respond to emailed questions about security for the ambassador.

By law, South Korea provides police protection to a U.S. ambassador only when the American Embassy requests it, according to South Korean police. The U.S. Embassy didn't request security for Lippert, but 29 officers were deployed as a precaution, although all but four were on standby outside the building, the officer said.

Even before the attack on Lippert, Kim had been well-known among police and activists as one of a hard-core group of protesters willing to use violence to highlight their causes. He received a three-year suspended sentence in 2010 for throwing a piece of concrete at the Japanese ambassador to Seoul while protesting Japan's claim to small disputed islands that are occupied by South Korea.

More recently, Kim had been under investigation by Seoul prosecutors after allegedly assaulting at least one public employee at an outdoor pop concert in January.

Officials at Seoul's Jongno police station said they were aware of Kim's violent history, but did not consider the possibility that he would show up at the breakfast meeting, despite his ties to the group that hosted it.

When Kim Ki-jong entered the hall where the attack happened, a police officer asked one of the event organizers whether Kim should be allowed to enter, Yun, the Jongno district police chief, said. The organizing official answered that Kim could enter because he was associated with an organization that had been invited to the meeting.

A security expert called the police inept.

"There is no excuse for allowing a blacklisted person to enter the venue for an event like this," said Yu Hyung-chang, a professor at the Kyungnam University in Changwon who served in South Korea's presidential secret service for 20 years until 2000. "If you are going to let him enter, then the very basic thing to do is to have an officer stay close to him at all times."

Yu says the police, knowing what they did about Kim, should have told the Embassy about the dangers and provided better security, especially since there had been a recent surge in anti-U.S. demonstrations.

While most South Koreans look at the U.S. presence favorably, America infuriates some leftists because of its role in Korea's turbulent modern history.

Washington, which backed the South during the 1950-53 Korean War against the communist North, still stations 28,500 troops here, and anti-U.S. activists see the annual military drills with Seoul as a major obstacle to their goal of a unified Korea.

"South and North Korea should be reunified," Kim shouted as he slashed Lippert with a 25-centimeter (10-inch) knife, police and witnesses said.

Lippert became ambassador last October, and is a popular figure. He's regularly seen walking his Basset Hound, Grigsby, near his residence, not far from where the attack happened. His wife gave birth here and the couple gave their son a Korean middle name.

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AP writer Hyung-jin Kim in Seoul contributed to this report.

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Follow Foster Klug, AP's Seoul bureau chief, on Twitter at twitter.com/APKlug, and Kim Tong-hyung at twitter.com/KimTongHyung