Authorities allowed the plot to proceed in order to build up enough evidence to charge the suspect with attempt.
Officials didn't say if the suspect had any ties to other Americans recently accused of trying to carry out attacks on U.S. soil, including alleged efforts in May by a Pakistan-born man to set off a car bomb near Times Square or another Pakistan-born Virginia resident accused last month in a bomb plot to kill commuters.
U.S. Attorney Dwight Holton released federal court documents to The Associated Press and the Oregonian newspaper that show the sting operation began in June after an undercover agent learned that Mohamud had been in regular e-mail contact with an "unindicted associate" in Pakistan's northwest, a frontier region where Al Qaida and Afghanistan's Taliban insurgents are strong.
The two used coded language in which the FBI believes Mohamud discussed traveling to Pakistan to prepare for "violent jihad," the documents said.
In June an FBI agent contacted Mohamud "under the guise of being affiliated with" the suspected terrorist. But the documents did not say how federal officials first became aware of Mohamud.
An undercover agent met with him a month later in Portland, where they "discussed violent jihad," according to the court documents. As a trial run, Mohamud and agents detonated a bomb in Oregon's backcounry earlier this month.
"This defendant's chilling determination is a stark reminder that there are people – even here in Oregon – who are determined to kill Americans," Holton said.
Friday, an agent and Mohamud drove to downtown Portland in a white van that carried six 55-gallon drums with detonation cords and plastic caps, but all of them were inert, the complaint states.
