Bomb robots were inspecting a van with an unconscious man inside, and a school as well as nearby residents were evacuated Tuesday in Corona, after a warning note was found on the van saying it would explode if moved.

Hundreds of students and dozens of staff members were evacuated from Parkridge Elementary School in Corona after the note was discovered on the van parked nearby, officials said.

At 10 a.m., an apartment manager noticed the van parked in the alley near the 800 block of Main Street. He saw a man sleeping inside.

"He found a note that was attached to the front windshield of that van and it indicated that there was dangerous gases inside the van, and the van would detonate and explode if the van was moved," Sgt. Paul Mercado of Corona police said.

Tactical robots opened the back doors to reveal blue containers and a man lying down inside the back of the van.

A backpack was removed from the van by the robots. The bomb threat was cleared by police just before 7 p.m. The condition of the man in the van remained unclear.

A total of 750 students and 80 staff members were evacuated from the school at 750 Corona Avenue, according to Lynn Mata of the Corona Fire Department.

Corona police also began knocking on doors around the neighborhood to evacuate residents.

Despite the inconvenience, residents and parents said they were glad the police were taking the incident seriously.

"Not knowing the situation that your son might be in school -- you're at home thinking they're safe, and they're not. It's very scary," Alma Diaz said.

Tony Shin contributed to this report.