Boeing's Everett plant was evacuated Thursday evening after police received unsubstantiated reports of a man with a gun on the premises.
Boeing tweeted from its official Twitter handle that the company's security and police investigated the potential threat.
"All precautions being taken until we know more," Boeing said in the tweet Thursday night.
Everett Police spokesman Aaron Snell said several Boeing employees reported seeing a man walking around the area with a rifle. Police said they searched the buildings but did not seen anyone with a gun.
There were no reports of shots being fired.
"At 7 p.m. a (Boeing) employee called 911 and told police he said had seen a male he believed was holding a rifle outside one of the buildings," Snell said Thursday. Shortly after that, police received a second call about a male possibly with a weapon.
" In this case, I believe employees saw something they believed was a rifle, and they made a very good choice to call 911," Snell said.
A Boeing spokesman said Thursday evening that " Boeing Security and police are investigating an unsubstantiated report. All precautions are being taken until we know more information."
The spokesman said Boeing does run three shifts at the Everett facility, but would not say whether any of the production lines were shut down as a result of the incident.
Everett, Marysville and Mill Creek law enforcement officers searched the building and the perimeter but ended the search at 9:15 p.m. after they found nothing.
Nearby Marysville Pilchuck High School was the scene of a deadly shooting last week when a high school student walked into the cafeteria and shot and killed two students and injured four others before taking his own life.
Boeing and Everett Police took Thursday night's report very seriously, Snell said, and interviewed the two Boeing employees who reported seeing the armed man. They were not able to find anyone matching the witnesses' descriptions, he said.
The Everett plant is the largest building in the world and is planning for a 200,000-square-foot expansion. The company assembles 767, 777, 787 and 747 aircraft at the plant.