A shooting at an elementary shchol as left two adults dead, an eight-year-old killed and one student critically wounded, according to the San Bernardino police chief, in what officials described as a murder-suicide.

San Bernardino Police Captain Ron Maass said the male gunman visited a female teacher in her classroom at North Park Elementary school when he opened fire and killed her. He wounded two students before turning the gun on himself.

The teacher was identified by law enforcement officials as Karen Smith and the suspected shooter was named as Cedric Charles Anderson, a resident of the City of Riverside, multiple law enforcement agencies confirmed to NBC4.

Officials are trying to find out if there was a relationship between the victim and killer.

The 8-year-old child was identified as Jonathan Martinez by San Bernardino police. He was taken to Loma Linda University Medical Centre where he later died.

San Bernardino Police Chief Jarrod Burguan tweeted that investigators believe "the suspect is down." He added that there was no further threat to the school, according to AP.

Two students were airlifted to a hospital after what is believed to be a domestic dispute, San Bernardino City Unified School District spokeswoman Maria Garcia told news station KNBC-TV.

"We believe the teacher knew who the shooter was," Garcia said.

A shooter walked into the grounds of North Park Elementary School on Monday (10 April) and opened fire, say officials.

Multiple victims were confirmed by 10.45am local time by the San Bernardino County Fire although the exact number of people hurt or killed was not released.

The shooter may be dead, according to a tweet from San Bernardino Police Department Chief Jarrod Burguan, who added that the information was preliminary, according to KTLA5.

"We believe this to be a murder suicide," he tweeted. "Happened in a class room. Two students have been transported to the hospital."

Eileen Hards, a spokeswoman with the San Bernardino Police Department, confirmed that a shooter had entered the school.

Cal State University San Bernardino students were told to seek shelter on campus as reports of an active shooter at the elementary school, which is located a a few blocks from campus, filtered through.

The San Bernardino County Fire Department reported multiple gunshot victims and a triage area had been set up. "It's a developing situation," said Eric Sherwin, a Fire Department spokesman.

The condition of the two victims was not immediately known, police said.

San Bernardino has seen a large increase in gun violence recently. There were 62 slayings in the area last year — a 41% increase from the year before – and the deadliest year in the city since 1995.