Police in Washington, DC said they had failed to locate a "lone gunman" reported earlier today, which caused the entire base to be put on lockdown.
The site remained locked down in the late afternoon after unconfirmed reports shots had been heard.
The area in on alert and police are on the scene of the incident, with roads blocked and a helicopter overhead, according to Associated Press.
There were no casaulties. Police said they had focused their search on building 197, which was the site of a mass shooting in 2013, when military contractor Aaron Alexis killed 12 civilian workers. He was eventually short by police.
Eyewitnesses said the yard was “in lockdown with local and federal law enforcement swarming all over the area".
“Uncomfirmed reports are that there is – again – an active shooter somewhere in the yard.”
Separately there are reports of a hoax incident in New Jersey.