MADISON, Wis. -

21 people were killed over the weekend in Afghanistan.

That's the deadliest attack against foreigners since this war started 13 years ago.

Lexie Kamerman, a teacher at the American University of Afghanistan, was among those killed.

Kamerman was 27 and from Chicago.

Kabul residents laid flowers at the site of the bombing.

A college friend of Kamerman, John Baillie, recalled one of the last times he spoke with her.

"I just said, 'I hope you'll be safe,'" said Baillie. "And I think that she was being safe, and it was just unfortunate because I'm sure she was happy. She was in a restaurant with a new friend, wining, dining, having a great time."

Both the United States and the Afghan government have condemned the attack.

The Taliban claim the suicide attack was retaliation for an Afghan military operation earlier in the week.