The survivors of one of the deadliest school shootings in modern American history are still processing the attack on Wednesday in Parkland, Fla., that left 17 dead.

One student at Marjory Stoneman Douglas High School had an encounter with the shooting suspect that left him reeling.

During his English class on Wednesday afternoon, Chris McKenna, 15, left to take a bathroom break.

The restrooms were closed on the first floor, said Chris, a freshman, so he headed to the second floor. He found the shooting suspect, Nikolas Cruz, 19, in a stairwell between the first and second floors in the freshman building.

Sitting on the stairs, Mr. Cruz was loading a semiautomatic AR-15 rifle, Chris said.

Mr. Cruz appeared “shocked” when he opened the door to the stairwell, Chris said. “I don’t think he expected me to see him at all,” he said, as Mr. Cruz was loading the gun.