CHICAGO - A police officer assigned to a northern Illinois high school shot and wounded a 19-year-old former student who had brought a gun to the school on Wednesday morning, authorities in the city of Dixon said.

The suspect, who was in police custody, suffered wounds that were not life-threatening, and no one else was hurt in the incident, officials said. Police believe the former student acted alone and that there was no further threat.

"Things could have gone much worse," Dixon Mayor Liandro Arellano Jr. told a news conference. "A lot of things went right today."

All schools in Dixon, a small city about 100 miles (160 km) west of Chicago where former US President Ronald Reagan lived as a boy, were placed on lockdown.

The male suspect fired several shots near the west gym of Dixon High School, and when confronted by Officer Mark Dallas, he exited the school and ran, Dixon Police Chief Steven Howell Jr said at the news conference.

With the officer in pursuit, the former student shot several rounds. The officer returned fire and hit the man, Howell said. Shortly afterward, the suspect, who was not identified, was taken into custody and was receiving medical attention.

The officer will be put on administrative leave, which is the department's normal policy, the police chief said.

"He saved an enormous amount of lives," Lee County Sheriff John Simonton said of Dallas.

His comments were echoed by Illinois Governor Bruce Rauner, who in a statement credited "school resource officer Mark Dallas for his bravery and quick action to immediately diffuse a dangerous situation".

The FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives responded and assisted local law enforcement agencies.

Responding officers found that students and staff had barricaded classroom doors with desks, bookcases and other objects as they learned in training, Howell said.

Officials said all schools in the city about 80 miles (130 kilometers) west of Chicago were placed on lockdown in the immediate aftermath of the shooting. The other schools reopened after officials determined the gunman acted alone.

Armed school resource officers have been in the headlines since the Feb 14 massacre of 17 teens and educators at a Parkland, Florida, high school, where an on-duty resource officer did not confront the gunman, a former student of the school.

Reuters-ap