A gunman armed with an assault rifle has killed a man and critically wounded two women in a California neighbourhood near two polling sites, before the attacker was found dead inside a nearby home, authorities say.

Key points: No indication shooting related to election day or the polling station: police

No indication shooting related to election day or the polling station: police Shooting victims include two females, two males, but no details of deceased confirmed

Shooting victims include two females, two males, but no details of deceased confirmed Park where a polling place is located is in lockdown

According to Los Angeles Country Sheriff's Lieutenant John Corina, the 45-year-old man was a former member of the military and a "gun fanatic".

He had been bingeing on cocaine at least a day earlier, prompting his wife and children to leave their home the evening before the incident, Lieutenant Corina said.

The shooting took place on the day Americans headed to the polls to elect their new president, Donald Trump.

One polling place was reopened hours after a gun battle involving the gunman and Azusa police, that started about 2:00pm (local time) near the site.

But it "had nothing to do with the ballot or voting," Lieutenant Corina said.

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"It just happened to be across the street from the park where the voting was taking place and also an elementary school" that was being used as a polling site," he said.

"Nobody was shooting at those locations," Lieutenant Corina said.

County election officials advised voters to visit other polling places where they could cast provisional ballots.

Police responding to reports of afternoon gunfire exchanged shots with the suspect before taking cover in the park, Azusa police Chief Steve Hunt said.

No officers were hurt.

A SWAT team sealed off the area and hours later entered the nearby home, where they found the gunman dead, authorities said.

Investigators did not know whether the gunman died by police bullets or killed himself, Lieutenant Corina said.

Police earlier said they were hunting a heavily armed woman.

Two women in their mid-50s were wounded and hospitalised in critical condition, Lieutenant Corina said.

The motive of the shooting was unknown and it was unclear whether the attacker knew his victims, Lieutenant Corina said.

The names of the gunman and victims were not immediately released.

SWAT team moves in on a house near where shots were fired in Azusa. ( AP: Leo Jarzomb/San Gabriel Valley Tribune )

AP/ABC/Reuters