DEKALB, Ill. — When Steven Kazmierczak strode from behind a screen onto the stage of a Northern Illinois University lecture hall carrying four guns, he saw rows of students lined up in an auditorium that sloped toward him.

It took no more than 90 seconds from the first 911 call for campus police to make it into the Cole Hall room, yet by the time they rushed in, three students and the gunman lay dead, two more were mortally wounded and terrified students were still bolting for the exits.

As bad as it was — 16 others were hurt — it quickly became clear that Thursday’s carnage could have been far greater.

Investigators recovered 48 shell casings and six shotgun shells, but also discovered that Kazmierczak had pouches of unspent ammunition around his waist and hadn’t unholstered one of his four weapons. University officials said the rapid response by campus police and plans put in place after the attacks at Virginia Tech also kept Kazmierczak from causing more harm.

“In spite of the enormity of this tragedy, it could have been worse,” NIU spokeswoman Melanie Magara said Friday.

The 27-year-old Kazmierczak was spotted Thursday afternoon walking the short distance between his car and Cole Hall carrying a guitar case; police determined that’s how he concealed the 12-gauge shotgun.

He entered through a back door, NIU Police Chief Donald Grady said, and moved forward, the shotgun announcing his purpose.

“The barrel was the first thing in the room,” said John Giovanni, 20, a student in the geology class.

Kazmierczak said nothing as he took a few steps toward the front of the stage, appearing neither hurried nor agitated. The instructor may have never even seen him.

“The teacher had his back to the students,” said George Gaynor, 23, who also was in the class.

Witnesses said Kazmierczak pumped the weapon, preparing to fire. He didn’t seem to be aiming at anyone or anything, Giovanni said. Instead, he just pointed the gun in the direction of the largest concentration of students in the middle of the room.

At 3:06 p.m., he pulled the trigger.

“He was shooting from the hip. He was just shooting,” said Giovanni, 20, of Des Plaines, who ran from the room so fast that he lost a shoe. “I was running but I was hurtling over people in the fetal position.”

Students jumped up and ran as the sound of the shotgun and handgun rang in their ears.

The chief, who led the first team of officers into the hall, said that a total of 10 officers were at the hall within 90 seconds, but that enough time had passed for Kazmierczak to reload the shotgun.