The Californian gunman, whom authorities identified as John Patrick Bedell, looked as if he was trying to gain entry into the US defence department headquarters when he started shooting as soon as he reached a security checkpoint, police said.
"There is no indication at this point that there (is) any domestic or international terrorism nexus to this at all," Richard Keevill, chief of Pentagon police said. "... There doesn't appear to be anyone else acting in concert with Bedell."
Citing ignorance about the motive behind the attack outside the Pentagon entrance, Keevil said that the authorities are examining Internet postings by a user identified as "JPatrickBedell" for clues.
The posting refer to the US marijuana laws and marijuana-related felony charges in 2006 in California against John Patrick Bedell, including the 9/11 attacks and case of a US Marine Corps colonel who died in 1991.
However, Keevill said that the postings are yet to be confirmed as that of the accused by the authorities.
Referring to the surveillance video for support, Keevill said that the video "pretty much confirms what we've been saying, that he acted alone."
Keevill said that the gunmen instead of presenting the Pentagon pass required to enter the building, pulled out a gun and fired.
"He just reached in his pocket, pulled out a gun and started shooting," Keevill said.
Two security officers were hurt in the shootout that took place at the security checkpoint outside the entrance of the Pentagon complex.
The gumen was fatally shot in the attack.
Pentagon was one of the targets during the Sep 11 2001 hijacked plane attacks. 125 people, along with the plane's 59 passengers and crew and the five hijackers were killed when on of the planes hit the building.