A gunman opened fire inside the US Holocaust Memorial Museum in Washington D.C., killing a security guard before being shot and injured. Authorities say the assailant was hospitalised in a critical condition.

The Washington Monument looms over the U.S. Holocaust Memorial Museum in Washington, Wednesday, June 10, 2009. [Charles Dharapak/AP Photo]

Police say the gunman began firing with a rifle as soon as he stepped into the museum. Security guards acted immediately to stop him.

A law enforcement official says James Von Brunn, 89, a white supremacist, is under investigation. The elderly man's car was found near the museum and tested for explosives.

Von Brunn has a racist, anti-Semitic web site. He also wrote a book entitled "Kill the Best Gentiles", alleging a Jewish "conspiracy to destroy the white gene pool.

In 1983, he was convicted of attempting to kidnap members of the Federal Reserve Board because of high interest rates and US economic difficulties.

The attack is the third in a recent wave of unsettling shootings that appear to have political underpinnings.

At the White House a few blocks away from the museum, the press secretary said President Barack Obama was obviously saddened by the violence.