Bales shrugged, the medic, Sgt. 1st Class James Stillwell, testified Tuesday.

“If I tell you, you guys will have to testify against me,” Stillwell quoted him as saying.

The statement was one of many attributed to Bales that suggest he knew what he was doing the night he surrendered after a two-village killing spree in southern Afghanistan, prosecutors say.

The remarks, offered by fellow soldiers testifying for the government Monday and Tuesday, could pose a high hurdle for defense lawyers who have indicated that Bales’ mental health will be a big part of their case. The testimony is part of a preliminary hearing being held to help determine whether the case goes to a court martial.

Defense lawyers have noted that Bales was serving his fourth deployment, and had suffered from post-traumatic stress disorder as well as a concussive head injury in Iraq. One witness testified Tuesday that he was quick to anger.

The 39-year-old father of two from Lake Tapps, Wash., faces 16 counts of premeditated murder and six counts of attempted murder in the March 11 attack on the villages of Balandi and Alkozai, which counted nine children among its victims.

One of the worst atrocities of the Iraq and Afghanistan wars, the attack prompted the U.S. to halt combat operations for days in the face of protests, and military investigators couldn’t reach the crime scenes for a month.

A prosecutor’s opening statement and witness testimony Monday suggested Bales spent the evening before the massacre at his remote outpost of Camp Belambay with two other soldiers, watching a movie about revenge killings, sharing contraband whiskey from a plastic bottle and discussing an attack that cost one of their comrades his leg.

Within hours, a cape-wearing Bales slipped away from the post and embarked on a killing spree of his own, said the prosecutor, Lt. Col. Jay Morse. He attacked one village then returned to Belambay, where he woke up a colleague and reported what he’d done, Morse said. The colleague testified that he didn’t believe Bales and went back to sleep.

Bales headed out again, Morse said, and attacked the second village, bringing his death toll to 16 before returning once again in the predawn darkness, bloody and incredulous that his comrades ordered him to surrender his weapons.

His return to the base was captured on surveillance video, Morse said.

Soldiers testified that after being taken into custody, Bales told them, “I thought I was doing the right thing.”

“It’s bad, it’s really bad,” he reportedly added.

And Stillwell, the medic, said Bales told him that the soldiers at Camp Belambay would appreciate his actions once the fighting season ramped up: “You guys are going to thank me come June.”

At another point, Bales remarked, “I guess four was too many” — an apparent reference to the number of family compounds in the attacked villages, Morse said Monday.

Bales was largely calm and compliant when he turned himself in following the massacre, several soldiers testified Tuesday. He followed orders and sometimes sat with his head in his hands, as though the magnitude of what he had done was sinking in, one said.

At one point, Bales made a joke — pointing his finger, in the shape of a gun, at two soldiers guarding him — in what they took as a failed effort to ease the tension.

But Bales also deliberately mangled his laptop, said two soldiers assigned to guard him as he gathered his things.

One of them, Sgt. Ross O’Rourke, testified that he removed the laptop from Bales’ rucksack after the defendant told him he didn’t want to take it with him. O’Rourke said Bales then grabbed the computer and folded the screen back, breaking it.

That didn’t damage the hard drive, O’Rourke said, and investigators still could have retrieved information from the computer. O’Rourke didn’t testify about what information might have been uncovered.

On Monday, Cpl. David Godwin testified that Bales asked him to bleach his blood-soaked clothes.

Two other soldiers, Pfcs. Derek Guinn and Damian Blodgett, testified Tuesday that they were on a guard shift early March 11 when they heard scattered gunfire coming from Alkozai, the first village attacked. They used thermal imaging and then shot up flares to illuminate the area, but couldn’t make out what was going on.

Blodgett said he reported it to the operations center on base, and a specialist told them to monitor it and let him know if it came toward them.

The shooting lasted for 30 to 40 minutes, Blodgett said.

Guinn said he considered Bales to be bipolar: “Sometimes he was in a really good mood, and he seemed really angry sometimes, or easily annoyed.”

Two other witnesses said that later, an interpreter arrived with two Afghan National Army soldiers who reported that they had seen an American come and go from the base. Guinn gave a slightly different account when he recalled that the interpreter said the soldiers had seen two Americans arrive on base, and one head back out.

After the shootings, some Afghan villagers questioned whether they could have been carried out by one soldier.

Bales has not entered a plea, and is not expected to testify. His attorneys, who did not give an opening statement, have not discussed the evidence, but say Bales has post-traumatic stress disorder and suffered a concussive head injury during a prior deployment to Iraq.

Bales has not participated in a medical evaluation known as a “sanity board,” because his lawyers have objected to having him meet with Army doctors outside their presence.

Bales’ lawyers called their first witness Tuesday, a soldier who bagged the blood-soaked clothes Bales had been wearing as evidence. The testimony focused primarily on how the evidence was handled.