When a mail clerk at American Century Investments, a Kansas City investment company, opened up a package sent to the company in late January, it read: "BANG YOU'RE DEAD."
Sources tell ABC News that the package contained a pipe bomb filled with explosives, along with a wiring-and-triggering mechanism to detonate the device.
The package had been sent by someone who called himself "The Bishop," and investigators are taking the threat very seriously.
"This case is our top priority. We are very serious about solving this case. We put all of our top people on this investigation," said Paul J. Trimbur, who heads the Liaison Group for the U.S. Postal Inspection Service.
The only thing the bomber left undone in the Kansas City device was connecting the trigger wires that would make the package explode upon opening, ABC News has learned.
The Bishop wrote in a letter that accompanied the device that it did not explode because he chose not make it operational. If his demands are not met, the next device, the bomber warned, will kill.
Fred Burton of Stratfor Security is deeply concerned that the bomber has become agitated and is serious about hurting someone.
The Bishop has been sending threatening letters to businesses in the Midwest since May 2005.
'We're Using Our Full Forensic Lab'
At a laboratory near Washington, D.C., postal service forensic examiners are in a race against time to stop a potential deadly bombing campaign.
They are examining not only the Kansas City pipe bomb, but another that was mailed to Chicago.
"We're using our full forensic lab out here. We're doing the handwriting analysis, the fingerprint analysis. … We do the tracing to test what kind of explosive was used," Trimbur said.
The Bishop is demanding that certain financial firms take action to force a number of stocks to a price of $6.66 per share, an apparent satanic reference.
Apparently angry at being ignored, the Bishop has sent letters that have become increasingly aggressive in tone.
In 2005, the Bishop wrote: "It is so easy to kill somebody, it is almost scary. Just think, it could be as simple as mailing a package just like The Unibomber use [sic] to."
The Bishop has also written about kidnapping someone or going on a shooting spree like John Allen Muhammad and Lee Boyd Malvo did in the 2002 Washington Beltway sniper shootings.
In June 2006, the Bishop angrily wrote, "TIMES UP. I have acquired targets" and warned that he would be sending "packages."
Tonight, several dozen postal inspectors and agents from the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives are desperately searching for leads.
Pierre Thomas filed this report for "Good Morning America."