The 28-year-old man, identified in a search warrant as Neil E. Prescott, told a supervisor at software and mailroom supplier Pitney Bowes that he wanted to see his boss' "brain splatter all over the sidewalk," according to a search warrant.
"I'm a joker and I'm gonna load my guns and blow everybody up," Prescott said, according to the warrant.
The threats were made repeatedly in two separate phone calls this week, and investigators who searched Prescott's apartment Friday morning found several thousand rounds of ammunition and about two dozen semi-automatic rifles and pistols. The weapons so far appear to have been purchased legally but are still being examined, said Mike Campbell, a spokesman for the Bureau of Alcohol, Tobacco, Firearms and Explosives.
Prescott was taken into custody at his apartment Friday and was receiving a psychiatric evaluation at a hospital. He was not expected to be charged Friday, the Prince George's County police department said on Twitter.
"We can't measure what was prevented here, but what was going on over the last 36 hours was a significant incident in the county. And we think a violent episode was avoided," said county police Chief Mark Magaw.
The workplace Prescott is accused of threatening to shoot up is located just outside Washington.
It wasn't immediately clear when the threat was to be carried out or how seriously it was, but last week's mass shooting at a Colorado theater during the latest Batman movie - coupled with the "Joker" reference - put police on edge and gave the comments extra urgency, officials said.
"We take all threats seriously. And if you're going to make a threat, we will take action," Magaw said.
Though there's no other indication of a link to the Colorado shooting, police believe the joker comments were a "clear reference" to the killings, the warrant said. The man accused in those shootings, James Holmes, had his hair dyed reddish-orange, and called himself the Joker, New York Police Commissioner Ray Kelly has said.
It was not immediately clear if Prescott had a lawyer. His record in Maryland appeared to include only a 2007 speeding ticket.
A search for Neil Prescott on a website that tracks users' online activities led to a profile that appears to be him on mdshooters.com, a website for gun enthusiasts. On it, Prescott, who used an online tag of slog403 and identified himself as from Crofton, traded advice with other users about obtaining firearms.
During a conversation last week about acquiring a 30-round magazine, he indicated he would "never violate MD laws as I respect this site and state." But in a July 18 post, he also said he wished to "unleash 30 rounds of hell" and added a smiley face emoticon. It wasn't clear what he was referring to.
He was taken into custody Friday morning at his apartment in Crofton, near Annapolis, after a supervisor reported the threat to the police. Police on Thursday made an initial visit to Prescott's home, where he appeared groggy and was wearing the T-shirt with the reference to guns.
"The multiple threats and the nature of the threats and that action, together, led us to a place where we could get an emergency petition" for a psychiatric evaluation, said Deputy Police Chief Henry Stawinski.
According to the warrant, Prescott made the threat during a phone call on Monday morning when a supervisor contacted him on a work-related matter, then made similar statements in a separate conversation about 15 minutes later. During the call, Prescott acknowledged it was "kind of foolish of me" to say this over the phone, the supervisor told authorities.
The supervisor did not want to talk to a reporter, his son said.
Pitney Bowes spokeswoman Carol Wallace said in a statement that Prescott was an employee of a subcontractor to the company and had not been on any Pitney Bowes property in at least four months.
Wilbert Brinson, who lives in a building across from Prescott's but did not know him, said he was alarmed by the alleged threats.
"It's an awakening, you know, after hearing what happened in Colorado," he said.
Associated Press writers Jessica Gresko and Brian Witte and AP researcher Judith Ausuebel contributed to this report. Zongker reported from Washington.