U.S. investigators believe the U.S. soldier accused of killing 17 Afghan civilians split the slaughter into two episodes, returning to his base after the first attack and later slipping away to kill again, two American officials said Saturday.

This scenario seems to support the U.S. government's assertion — contested by some Afghans — that the killings were done by one person, since they would have been perpetrated over a longer period of time than assumed when Army Staff Sgt. Robert Bales was detained March 11 outside his base in southern Afghanistan.

But it also raises new questions about how Bales, who was formally charged Friday with 17 counts of premeditated murder and other crimes, could have carried out the nighttime attacks without drawing attention from any Americans on the Kandahar province base.

The two American officials who disclosed the investigators' finding spoke on condition of anonymity because the politically sensitive probe is ongoing.

Many details about the killings, including a possible motive, have not been made public. The documents released by the U.S. military Friday in connection with the murder charges do not include a timeline or a narrative of what is alleged to have happened.

Bales, 38, is accused of killing nine Afghan children and eight adults. The bodies were found in Balandi and Alkozai villages -- one north and one south of the base, in Kandahar's Panjwai district.

Bales also was charged with six counts of attempted murder and six counts of assault in the same case.

Bales slipped off Kandahar base twice, investigators believe

U.S. investigators now believe that Bales walked off his base that night and killed several people in one of the villages, then went back to the base. The American officials, who are privy to some details of the investigation, said they do not know why Bales returned, how long he stayed or what he did while there.

He then slipped off the base a second time and killed civilians in the second village before again heading back toward the base. It was while he was returning the second time that a U.S. military search party spotted him. He is reported to have surrendered without a struggle.

Bales is being held in a military prison at Fort Leavenworth, Kan.

There have been previous suggestions that Bales could have returned to base after the first set of shootings, but the American officials who spoke to The Associated Press on Saturday provided the first official disclosure that U.S. investigators have come to this conclusion.

Members of the Afghan delegation investigating the killings said one Afghan guard working from midnight to 2 a.m. on March 11 saw a U.S. soldier return to the base around 1:30 a.m. Another Afghan soldier who replaced the first and worked until 4 a.m. said he saw a U.S. soldier leave the base at 2:30 a.m. It's unknown whether the two Afghan guards saw the same U.S. soldier.

U.S. officials have said Bales left the base the first time armed with his 9mm pistol and M-4 rifle, which was outfitted with a grenade launcher.

Mental state to play role in case, lawyer says

Bales's civilian attorney, John Henry Browne, said Friday that he believes the government will have a hard time proving its case and that his client's mental state will become an important issue. Browne has said Bales suffered from the stress of serving four combat tours.

The decision to charge Bales with premeditated murder suggests that prosecutors believe they have sufficient evidence that he consciously conceived the killings.

The maximum punishment for a premeditated murder conviction is death. The mandatory minimum sentence is life imprisonment with the chance of parole.

Legal experts have said the death penalty would be unlikely in the case. The military hasn't executed a service member since 1961 when an army ammunition handler was hanged for raping an 11-year-old girl in Austria. None of the six men currently on death row at Fort Leavenworth was convicted for atrocities against foreign civilians.

The pre-dawn shooting spree has further frayed ties between U.S. troops and President Hamid Karzai as the two nations are negotiating agreements for America's military footprint in Afghanistan after most international combat forces withdraw by the end of 2014. After the shootings, Karzai reiterated his demand that foreign troops leave posts near Afghan villages and pull back to larger bases.

The killings also have fuelled anti-American sentiment in a country where violent protests raged for nearly a week last month after Muslim holy books and other Islamic texts ended up in a garbage burn pit at a U.S. base.

"By the laws of Islam, the soldier should be hanged," Dost Mohammad, a shopkeeper in Kandahar, the provincial capital, said Friday. "While other countries' laws say that this individual should be given a life sentence, from our point of view and that of other Muslims, this guy should be hanged."

Afghan officials and villagers maintain that 16 civilians were killed. The U.S. military never announced a death toll, but said Friday that investigators had collected enough evidence to charge Bales with killing 17 civilians.

U.S. officials are working with Afghan officials to compensate relatives of the victims, money that likely would be disbursed to the eldest male of the family. Eleven of those who died were from one family.