About a dozen Under Armour workers were evacuated after the call came in at 2:18 p.m. Sunday, police said. Police barricaded streets surrounding the company's campus for hours while a SWAT team swept each building room by room.
Investigators said they believe the caller was not an Under Armour employee and had no association with the company. Police said they were determined to make an arrest in the case.
"We're going to make sure somebody goes to jail," police spokesman Eric Kowalczyk said, noting that significant police resources were used investigating the threat. "This call put lives at risk."
Kowalczyk said police generally don't comment on how many officers are used in such responses. It wasn't clear how much the hoax call cost taxpayers.
Under Armour sent an email to employees Sunday evening to tell them the company would be open Monday morning.
Jeff Doyle, who has worked at Under Armour's call center for two years and lives nearby in Locust Point, heard about the situation via text messages from co-workers. He saw police cars outside and went out to see what had happened.
The police presence was unusual, neighbors said, for what they consider one of Baltimore's safest neighborhoods.
In August, Oriole Park at Camden Yards was the target of bomb threat that turned out to be hoax. As in the Under Armour case, the call to the ballpark came on a day when most of the stadium's employees and visitors were not there. The anonymous caller warned of an explosive device in a bathroom, but police found no evidence of a bomb.
Police wouldn't say whether the two calls are connected.
Ryan Kirby, of Bel Air, had parked next to one of the Under Armour buildings Sunday afternoon before heading to the Ravens-Bengals game; he returned to find a police barricade between him and his car.
Kirby, a junior at the University of Maryland, had left the football game at the end of the third quarter to make it back to College Park in time to finish a 10-page criminal-justice paper.
Instead of witnessing a thrilling, overtime Ravens victory, he and a friend sipped Royal Farms coffee and waited with a handful of Locust Point residents for officers to wave them through.
The prospects for Kirby's paper weren't looking good as the hours passed.
"I just hope my teacher understands the situation," he said.