Four more explosive devices tied to the suspect in the Discovery Channel standoff were found at a residence in Montgomery County, Md., authorities told NBC News on Thursday.
Authorities said a bomb squad detonated the devices. No one was injured.
Investigators are still trying to find the car they believe that the suspect, identified as James J. Lee, was driving on Wednesday before he entered the headquarters of Discovery Communications in Silver Spring, Md., with bombs strapped to him and took three people hostage.
Montgomery County Police Chief Thomas Manger said the gunman was shot to death about 4:50 p.m. EDT when authorities watching security cameras saw the hostages move in a possible run for freedom and heard a "pop" they believed to be a gunshot or an explosive going off.
"For most of the time, the hostages were laying on the ground, and he didn't engage them much other than saying on the phone, 'I don't care about these people,'" Manger said.
Manger said police planned to negotiate as long as necessary, but acted when officers believed the hostages were trying to escape.
"We saw some hand movements from hostages," Manger said. "We're watching this and trying to keep the focus of the guy we're talking to. We didn't know if they were up to something or what they were doing. We are surmising they were making a break for it."
Lee repeatedly said he was "ready to die," the police chief said.
"I believe he was not intending to come out of there alive," Manger said.
The hostages — two Discovery Communications employees and a security guard — were unhurt after the four-hour standoff.
Police say a review of Discovery Channel surveillance video indicates that the suspect may not have been wearing any explosive devices when he entered the building on Wednesday. It's possible, they say, that he had them in a box he carried and put them on once he was inside the building.
Officers scouring the building overnight found several devices on the suspect, including two table-top propane gas canisters and two pipe bombs, NBC News reported Thursday.
As for how powerful they were, investigators say they can't be certain, but they believe the device Lee was wearing would have killed him and could have harmed any hostages near him.
Also, Montgomery County, Md. police sources said the guns the hostage-taker had were only starter pistols, which can make noise but can't cause serious injury.
An NBC News producer who called the building to find out what was going on had a brief telephone conversation with the man when he came on the line unexpectedly. He said, "I have a gun and I have a bomb. ... I have several bombs strapped to my body ready to go off."
NBC News informed Montgomery County authorities of the conversation as the producer spoke to the man for about 10 minutes. NBC News did not report the conversation until the hostage situation had been resolved.
When Lee pleaded guilty in 2008 to disorderly conduct outside the Discovery building, he was ordered to stay away from it as a condition of his probation. That order, county officials say, expired two weeks ago.
The case stemmed from his protests against Discovery Communications, which includes the Discovery Channel, TLC, Animal Planet, Science Channel and Planet Green networks.
Lee appears to have , saying humans are ruining the planet and that Discovery should develop programs to sound the alarm.
Court records show that Lee was arrested Feb. 21, 2008, on the sixth day of a protest at the Discovery building. At the time of his conviction in March 2008, he was identified as being from San Diego.
Police were called to the scene when a crowd that had gathered began growing "unruly" as Lee threw thousands of dollars of cash into the air, some of it still in shrink-wrapped packages, police said at the time. (Lee was found not guilty of littering.)