Eight American troops and a U.S. contractor died Wednesday after an Afghan military pilot opened fire during a meeting at Kabul airport — the deadliest episode to date of an Afghan turning against his coalition partners, officials said.

The Afghan officer, who was a veteran military pilot, fired on the Americans after an argument, the Afghan Defense Ministry said.

All nine killed were American, according to a senior U.S. defense official, who spoke on condition of anonymity because the information has not yet been made public.