Staff Sgt. Robert Bales' plea ensures that he will avoid the death penalty for the middle-of-the night slayings that so inflamed tensions with the people of Afghanistan that the American military suspended combat operations there.
Prosecutors say Bales slipped away before dawn on March 11, 2012, from his base in Kandahar Province. Armed with a 9 mm pistol and an M-4 rifle equipped with a grenade launcher, he attacked a village of mud-walled compounds called Alkozai, then returned and woke up a fellow soldier to tell him about it.
The soldier didn't believe Bales and went back to sleep. Bales then left to attack a second village known as Najiban.
Relatives of the dead were outraged at the idea that Bales could escape execution when they spoke to The Associated Press in April in Kandahar.
"A prison sentence doesn't mean anything," said Said Jan, whose wife and three other relatives were slain.
A jury will decide in August whether the soldier is sentenced to life with or without the possibility of parole. He would serve his prison sentence at Fort Leavenworth, the military prison in Kansas.
"My last request from the world, all the countries, is that as a family of the victims we want our killer to be hanged,” said Haji Baran, whose brother was killed in the massacre.
The US military has traditionally been slow to convict service members or crimes against foreign citizens. No service member has been executed since 1961.