Ford's Kansas City assembly plant in Claycomo, Missouri was evacuated on Tuesday after a report of an active shooter, but officials have determined that the threat was not credible.
However, authorities were not able to locate anyone during an initial search of the area where the shooter was suspected to be, the Clay County Sheriff's office said in an update late on Tuesday.
There were no reports of any deaths, injuries or damages. The Clay County sheriff's office cautioned that it could possibly have been a "swatting call," and that its response team was entering the plant to locate the source of the threat.
People who make a swatting prank call try to trick emergency services into sending police, usually a Special Weapons and Tactics (SWAT) team, to respond to the false emergency.
A man called Ford's safety and risk management team at about 5:20 pm local time and claimed he was armed with explosives, a rifle, and a handgun, the sheriff's office said, adding that he claimed to have been barricaded in the second-floor bathroom in the paint area of the plant.
Ford has ceased all production, including for Tuesday's night shift, and has evacuated the plant, the sheriff added.
The Kansas City assembly plant employs about 7,250 workers, according to Ford's website.
Monitoring the situation
The United Auto Workers (UAW) union said it was closely monitoring the possible active shooter situation.
"It's our understanding that one individual is heavily armed inside the assembly plant, but swift action by union and plant leadership cleared the building," the union said in its statement.
There is now a large law enforcement presence at the facility, the statement said, adding that there were no reports of workers being in harm's way.
Ford did not immediately respond to a Reuters request for comment.