Two teenage attackers fatally shot three men at a mosque in San Diego, California, in a suspected hate crime, before taking their own lives, say police.

The assault took place on Monday morning as officers were investigating a call about a possibly suicidal teen who had run away from home.

Police were alerted to the shooting at the Islamic Centre of San Diego and found three victims with gunshot wounds outside the front of the building.

Shortly afterwards, they received another call that shots had been fired nearby from a vehicle at a landscaper. Officers found the suspects - aged 17 and 18 - dead of self-inflicted wounds in a vehicle blocks away from the mosque.

Among the deceased victims was a security guard who worked at the centre and "played a pivotal role" in preventing the attack from being "much worse", officials said.

"It's fair to say his actions were heroic," San Diego Police Chief Scott Wahl told a news conference. "Undoubtedly, he saved lives today."

Authorities have not yet identified the three victims by name. But the security guard was a father of eight, a person who knew him told the BBC's US partner CBS.

Investigators said the motive for the attack was unknown, but it was presumed to be a hate crime because of the mosque, which is the largest in San Diego County, and because of writings attributed to one suspect.

Police were first called to the mosque at 11:43 local time (18:43 GMT) and "observed what appeared to be three deceased victims out front", Wahl said.

"There were no officers involved in firing their weapons," Wahl said, and there was no sign of any gunman.

About two hours before the attack, the mother of one of the suspects had called police to report that her son had left home with several of her guns and her car.

The woman said he had gone with a companion, and both were dressed in camouflage.

Wahl said police found the suspect's behaviour to be "not consistent" with someone who is considered suicidal.

A note the youth left behind also included "generalised hate rhetoric and hate speech", he said.

Wahl added that the note contained no specific threat to the mosque or to any other location or individual.

Investigators went to a local high school, where one of the teens was a student, and to a shopping mall where the car had been tracked.

When the shooting took place, officers were still speaking to the mother and were only a few blocks away from the mosque.

Those officers, upon finding the three victims outside the building, rushed inside and began following active shooter protocols.

While they were clearing rooms, more reports came in of another shooting nearby.

The suspects had opened fire from their car at a landscaper, who was uninjured, police said.

Wahl said a bullet may have deflected off the landscaper's hard hat, although this had yet to be confirmed.

When police arrived at the second scene, a few blocks away from the mosque, they discovered the dead bodies of both suspects.

Children were in class as the incident unfolded on Monday. The Islamic Centre campus houses the Al Rashid School, which offers religion and language courses.

Aerial video from the scene on Monday showed children holding hands and being escorted through a car park at the centre as police responded.

Nearby schools were also placed on lockdown.

The FBI appealed to the public for any information that could help the investigation.

A witness speaking to CBS said he heard up to 30 gunshots from what sounded like "a semi-automatic weapon".

He said he first heard about a dozen shots, then a pause, then another possibly dozen shots.

The man, who is retired and was eating lunch at home, said he called 911 and that police arrived within "five to 10 minutes".

Imam Taha Hassane, director of the Islamic Centre of San Diego, said at a news conference: "It is extremely outrageous to target a place of worship."

The facility "is a house of worship, not a battlefield", he added.

The Muslim community is currently preparing for one of its holiest seasons and its biggest feasts.

It's days before Eid al-Adha, or the Festival of Sacrifice, which commemorates the obedience of Prophet Ibrahim.

California Governor Gavin Newsom released a statement that he was "horrified by today's violent attack" at the centre, "where families and children gather, and neighbors worship in peace and fellowship".

The state "will not tolerate acts of terror or intimidation against communities of faith", Newsom added.

Asked about the shooting on Monday, US President Donald Trump called it a "terrible situation".

"I've been given some early updates, but we're going to be going back and looking at it very strongly," he said during an unrelated White House event.