Despite high tensions with North Korea, U.S. government experts believed security for the U.S. ambassador to South Korea was adequate before he was slashed by a knife-wielding attacker Thursday, the State Department said on Friday.

Ambassador Mark Lippert needed 80 stitches after his face was cut at a forum discussing Korean unification in Seoul. His assailant had made multiple visits to North Korea between 1999 and 2007.

South Korean police said they were investigating possible links between the attack and the visits, but State Department spokeswoman Marie Harf said the motive was still not known. "We are still getting all the facts about how people were screened to get into this event, how they were on the list, how they were allowed in," she told a regular news briefing, adding that the embassy was coordinating with the Korean-led probe.