Two teenagers opened fire at a San Diego, California, mosque on Monday and killed three men before killing themselves a few blocks away, police said.

The attack at the Islamic Centre of San Diego is being investigated as a hate crime, San Diego Police chief Scott Wahl said at a news conference at a neighbourhood park across from the mosque.

He added the “circumstances that led up to this” would come out in the days ahead.

The actions of a “heroic” security guard who was killed in the incident saved lives, he said.

Wahl said later that officers had been talking to one of the teenagers’ mothers earlier in the day, after she had contacted police around 9.40am local time to say her son was missing along with multiple weapons and her vehicle.

“She was beginning to develop a bigger picture as to what she was dealing with and was conveying that to our folks, and we were trying to put that together and get out in front of this as quickly as we could,” Wahl said.

The mother also found a note, Wahl said. He did not disclose its contents but said of the case: “There was definitely hate rhetoric that was involved.”

He added there was no specific threat against the Islamic Centre of San Diego.

Wahl said the teenager whose mother contacted police was a student at Madison High School, about a mile away from the Islamic Centre. Police have not released the names of the teenagers.

Among those killed was a mosque security guard, who police believe “played a pivotal role” in keeping the attack from being “much worse”, the police chief said said.

Later he said authorities continued to review the security guard’s response, but “it’s fair to say his actions were heroic”.

“Undoubtedly he saved lives today,” Wahl said.

The centre is the largest mosque in San Diego County and includes the Al Rashid School that offers courses in Arabic language, Islamic studies and the Quran for students ages five and up, according to its website.

“All of the kids are safe,” Wahl said, appearing emotional. “Our hearts go out to the families that are in this moment being notified of what has happened to their loved ones.”

Police responded within four minutes of being called, according to police. As they arrived, gunshots rang out a few blocks away where a landscaper was shot at but uninjured.

The gunmen were found dead in a vehicle stopped in the middle of a road nearby.

Aerial TV footage showed more than a dozen children holding hands and being walked out of the parking lot of the centre as it was surrounded by scores of police vehicles. The white mosque is in a neighbourhood of homes, apartments and strip malls with Middle Eastern restaurants and markets.

Parents were directed to a nearby area to retrieve their children.

The mosque’s director, Imam Taha Hassane called it “extremely outrageous to target a place of worship”.

“All the places of worship in our beautiful city should always be protected,” he said.

He added that the centre focused on interfaith relations and community building, and that a group of non-Muslims had been touring the mosque earlier on Monday to learn about Islam.

The Islamic Centre’s website says its mission is to not only serve the Muslim population but also “work with the larger community to serve the less fortunate, to educate, and to better our nation”. Five daily prayers are held there, and the mosque works with other organisations and people of all faiths on social causes.

The Council on American-Islamic Relations (CAIR), one of the largest Muslim civil rights and advocacy groups in the US, condemned the shooting.

“No one should ever fear for their safety while attending prayers or studying at an elementary school,” said CAIR-San Diego executive director Tazheen Nizam in a statement.

“We are working to learn more about this incident and we encourage everyone to keep this community in your prayers.”

Governor Gavin Newsom’s office said he was being briefed.

“We are grateful to the first responders on the scene working to protect the community and urge everyone to follow guidance from local authorities,” his office posted on X. – Associated Press