MIAMI — Armed with an AR-15 assault rifle and more than 300 rounds of ammunition, Nikolas Cruz killed 17 people at Marjory Stoneman Douglas High School in Parkland, Fla., in February without entering a single classroom.

Instead, Mr. Cruz, a former Stoneman Douglas High student, carried out his carnage by walking down the hallways of the freshman building and taking aim at students and teachers trapped in the corridors or locked inside classrooms. Several times, he returned to victims he had already wounded to shoot them dead.

That was the chilling narrative that law enforcement provided on Tuesday in a minute-by-minute animation of Mr. Cruz’s movements through the school, the first time the police made public a detailed timeline of the gunman’s actions inside the building. The animation, played for members of a Florida commission investigating the mass shooting, showed that the gunman had time to pursue victims on all three floors of the building during his six-minute rampage.

At no point during the shooting did police officers enter the building or engage the gunman, even though there was an armed deputy from the Broward County Sheriff’s Office outside less than two minutes after the shooting began, and several other officers heard gunfire after they arrived. The law enforcement response is expected to be closely reviewed by a special public safety commission created by the Florida Legislature last month. Among its 16 members are three fathers of students killed in the shooting.