WASHINGTON - US investigators Friday sought to uncover why a California man opened fire outside the Pentagon before being fatally shot by police, as Internet posts suggested he feared a vast government conspiracy.

Police said 36-year-old John Patrick Bedell acted alone and appeared to have no links to terror groups when he opened fire at close range on police officers late Thursday near the entrance to the giant Defense Department building.

Bedell, a former electrical engineering student, was shot in the head by the officers, two of whom suffered minor wounds.

A surveillance video showing a well-dressed Bedell approaching the entrance "pretty much confirms what we've been saying, that he acted alone," Pentagon Force Protection Agency chief Richard Keevill told reporters.

"There is no indication at this point that there are any domestic or international terrorist" connections in the case, Keevill said.

He said no motive had yet been found for the assault that occurred during the evening rush-hour outside the metro system's busy Pentagon station.

The Federal Bureau of Investigation and other agencies were looking at Internet postings suggesting Bedell may have resented the US government and promoted conspiracy theories about the September 11, 2001 attacks.

"Investigative efforts include reviewing Bedell's possible Internet activity, his cell phone usage and other information which might assist in determining Bedell's state of mind at the time of the shooting," the FBI and police said in a statement.

Bedell, from the small California town of Hollister, reportedly studied electrical engineering at San Jose State University, where one of his professors was impressed with his intellect and saw no sign of any troubles.

"He was always on time. Always prepared. Always asking that great question that sparked further discussion," Professor David Parent told the San Jose Mercury News.

Bedell was accepted into the graduate electrical engineering program in 2008 and was last enrolled in the autumn of 2009, the newspaper said.

A different portrait emerged from Internet posts under his name, with rants about the "barbaric" US government, opposing laws banning marijuana use and warning of a coming "new dark age."

In one entry, user JPatrickBedell wrote he was working toward "establishing the truth of events such as the September 11 demolitions."

"I am determined to see that justice is served in the death of Colonel James Sabow," a US Marine who died in his backyard in 1991, he added.

The death was ruled a suicide but some followers of the case allege Sabow was murdered in a cover-up.

An audio file apparently from Bedell describes the US government as a "coup regime" run by an "international criminal conspiracy" that allegedly seized power after the 1963 assassination of former president John F. Kennedy.

One US law enforcement official, speaking on condition of anonymity, told AFP there were initial signs Bedell was mentally "unstable" and had run-ins with police.

Bedell "had a couple of contacts with the law" in the past, Keevill said, without providing more details.

Bedell's family issued a statement saying they were "devastated" by the news.

"We may never know why he made this terrible decision. One thing is clear though -- his actions were caused by an illness and not a defective character."

Keevill said Bedell had driven across the country from the west coast over the past few weeks and left his Toyota in a parking garage near the Pentagon. Police found more ammunition in the car.

Keevill said Bedell appeared quietly determined to inflict bloodshed at the center of US military power.

"He was very well armed," with two 9-millimeter semi-automatic weapons and several magazines, the police chief added, noting a "significant number of rounds" were fired in a close-range exchange that lasted less than a minute.

"He walked very directly to the officers and engaged" them in a shootout at a main entrance to the building, sending employees diving for cover, Keevill said.

Three officers shot back at Bedell. The officers were wearing bullet-proof vests and sustained minor injuries, with one grazed on the shoulder and the other on the thigh. They were later released after being treated in hospital.

After a hijacked plane slammed into the Pentagon in the 9/11 attacks, the building was fortified and the metro exit rebuilt so riders exit away from the Pentagon entrance.