It said Mohamud was warned several times about the seriousness of his plan, that women and children could be killed, and that he could back out, but he told agents: "Since I was 15 I thought about all this" and "It's gonna be a fireworks show … a spectacular show."
Mohamud, a naturalized U.S. citizen living in Corvallis, was charged with attempted use of a weapon of mass destruction and is scheduled for a court appearance Monday. Few details were available about him late Friday.
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.