A gunman killed two people and injured another at a library in Northern California on Monday before being taken into custody, the police said.

Just past 5 p.m. on Monday, the police in Chico, Calif., about 90 miles north of Sacramento, responded to reports of an active shooter inside the city’s Butte County Library branch, in a residential area less than half a mile from an elementary school.

The library was open during the shooting. On a 911 call, officers heard gunfire and screams, Chief Billy Aldridge of the Chico Police Department said during a news conference on Monday night.

“The incident tonight was obviously very sad and very traumatic for a lot of people — very traumatic for our community,” Chief Aldridge said, adding that he could not recall seeing anything like this in his time as chief.

The names of the two people killed, both adults, have not been released yet. The person who was injured, a juvenile, was taken to a hospital to receive treatment for a minor injury, Chief Aldridge said. The name of the suspect in custody has also not been released.

The gunman used one firearm inside the library before trying to flee out the back door, where officers were stationed, Chief Aldridge said. The F.B.I. and several local authorities, including the Chico Fire Department and the Butte County Sheriff’s Office, responded to the scene, he added.

The Chico police are investigating the shooting with the assistance of “multiple agencies” and will assess whether the gunman acted independently, Chief Aldridge said. “We don’t feel like there’s any further threat to the public,” he added.

Mollie Macarthy-Openshaw, a Chico resident who lives down the street from the library, had considered returning the books her grandchildren had checked out, but decided against it.

Instead, she took her dogs for a walk — and sprinted home when she heard a round of gunfire nearby.

“Praise the Lord I chose not to go to the library,” Ms. Macarthy-Openshaw, 63, said.

“My thoughts and prayers go out to the people who have perished and their families,” she added.

Heavy police presence surrounded the library around 7 p.m. on Monday. Streets were expected to remain closed to the public “late into the night,” Chief Aldridge said.