They appeared to be arguing when Amir Amin first saw them. He heard one of them yell, “Pick up the pace.”

“Then they just started firing,” the 18-year-old said Wednesday, one day after seven terrorists massacred at least 148 students and teachers in this northwestern Pakistani city, prompting a new round of soul-searching and forcing the government to shift tactics in its campaign against Islamist militants.

Amin, a dozen classmates and four teachers slipped into a classroom and locked the door. They sat on the floor, holding hands. “But the terrorists broke down the door, and it was like we got killed on the spot,” said Amin, who was shot in the hip. “Only two of us survived.”

In the aftermath, the gut-wrenching horror that Amin and hundreds of others at the Army Public School and Degree College faced is palpable.

Where the first shots were fired — in the main auditorium — pools of drying blood stain the floor near exit doors.

A picture hangs on a bullet ridden wall of Army Public School that was attacked by the Taliban militants in Peshawar, Pakistan, 17 December 2014. Pakistan began three days of mourning on 17 December for the 132 children and nine school staff killed in the deadliest ever attack by the Taliban. (Sohail Shahzad/European Pressphoto Agency)

Scattered shoes and broken eyeglasses — perhaps belonging to students who escaped or the scores who died — hint at the frantic and desperate moments after Taliban gunmen burst into the school.

A few yards away, computers lie shattered. Bookshelves are overturned. Door frames and windows in classrooms are ripped from their hinges.

The adjoining administrative offices reveal more signs of the mayhem: windows blown out, ceilings collapsed, walls charred.

And on school grounds, two human feet sit atop other body parts collected and placed on a white blanket.

In response to the carnage, Prime Minister Nawaz Sharif pledged Wednesday to pursue the militants beyond Pakistan’s borders and lifted a moratorium on the death penalty.

A high-level Pakistani delegation, including the army chief, rushed to Kabul to appeal for help against the terrorists, whose strongholds straddle the Pakistan-Afghanistan border. A military statement said Afghan President Ashraf Ghani promised to act to prevent Afghan soil from being used for cross-border militant activities.

In Peshawar, in an effort to show the world the magnitude of Tuesday’s assault on the school, Pakistani military leaders led journalists on a 90-minute tour of where the victims were killed. The attack also injured more than 100.

Peshawar, Pakistan.

The tour began with a news conference on the steps of the school. Maj. Gen. Asim Bajwa, chief spokesman for the Pakistani military, warned the journalists that they were about to see the “unexplainable.”

“Today is one of the saddest days of our history,” Bajwa said as he stood on the blood-stained steps. “Unfortunately, all of us are ashamed that we have these people with worse-than-animal instincts who could come and kill innocent children in such a mass scale.”

A day earlier, the militants used the same 18 steps to gain access to the school auditorium about 10 a.m.

They burst into the auditorium as about 250 students were receiving first-aid training from two army doctors. Bajwa said at least two of the gunmen climbed onto the stage and immediately began firing on the students in the movie-theater-style seats below.

Some students tried to escape through two side exits, but they were blocked by the other terrorists.

“And then they shot many of them point-blank in the head,” Bajwa said, noting the inch-thick puddles of blood near the exit doors.

In all, about 100 students died in the auditorium. On Wednesday, many of their books remained on the seats.

Bajwa said one teacher ran onto the stage to try to “prevent them from killing students.”

The militants, he said, apparently set the teacher on fire and “burnt her right there on the stage.”

A student recovering at nearby Lady Reading Hospital confirmed the account. Mehran Khan, 14, said he looked up after being shot in his arm, leg and back and saw “flames coming out of the body” of the teacher.

He survived because of his quick thinking. “I fell over onto the floor and acted like I died,” he recalled.

The terrorists then made their way to the nearby classrooms. Many of the students had already locked themselves in the rooms, but the terrorists used explosives and their feet to blast and kick their way in.

“They just started brutal firing,” Zulfiqar Ahmed, a math teacher who was shot four times, recalled from his hospital bed. “There were some 18 students with me in the classroom. All 18 got killed.”

The terrorists then moved into the administrative offices of the school, which is financed by the Pakistani military.

Bajwa said Principal Tahira Qazi, who had stayed in her office after the shooting began to field calls from terrified parents, eventually locked herself in the bathroom. But one of the militants lobbed a grenade into the room through a vent, killing the 60-year-old. Qazi was married to a retired army colonel.

Bajwa said the attackers started blowing themselves up as soldiers began advancing.

The blasts occurred with such force that several offices caught fire. But many “Best Sportsmen” and “Roll of Honor” placards on the walls escaped unscathed.

On Wednesday, the clock in the teachers’ lounge was stuck at 3:15 p.m. The clock had stopped working about five hours after the gunmen stormed the auditorium.

On the grass outside the administrative wing, soldiers have arrayed a 100-foot row of students’ and teachers’ belongings.

Cellphones. Backpacks. Purses. Wristwatches. Necklaces. Ties. T-shirts.

“No one wants to lose their children,” Bajwa said. “This is the worst example of being a human being. . . . We are all sad. We are all angered. We are all sickened.”

Haq Nawaz Khan and Aimar Iqbal contributed to this report.