US Holocaust Memorial Museum security guard Stephen Tyrone Johns has died after being shot by 88-year-old Maryland man James W von Brunn earlier today.

Von Brunn, who authorities say is a known white supremacist, reportedly walked into the museum at 12:50pm and shot Mr Johns with a shotgun as he manned a security checkpoint.

Two other security officers at the museum returned gunfire wounding von Brunn before police and emergency service responders could arrive on site.

A spokesman for the museum, Andy Hollinger, said in a statement that after von Brunn opened fire, “two museum security officers returned fire hitting the assailant.”

Sergeant David Schlosser, a spokesman for the US Park Police, had earlier said von Brunn and Mr Johns had been transported to George Washington University Hospital with serious injuries.

An eyewitness said she heard five shots and saw a security guard lying wounded on the ground.

“It was definitely a security guard, he was down bleeding on the floor,” she said. “He was face down. His back … blood was coming out.”