Mohamud is accused of plotting to carry out a terror attack Friday on a crowd of thousands at Portland's Pioneer Courthouse Square.
Officials said Mohamud had no formal ties to foreign terror groups, although he had reached out to suspected terrorists in Pakistan.
Authorities said Somalis in the United States have expressed concerns about tactics used by federal agents in the sting operation against Mohamud.
An FBI affidavit said agents began investigating after receiving a tip from an unidentified person who expressed concern about Mohamud.
An agent corresponded with Mohamud via e-mail and pretended to be affiliated with one of the people overseas whom Mohamud had tried to contact. Agents also met the suspect face-to-face at hotels.
Authorities said they allowed the plot to proceed to gather evidence to charge Mohamud.
Mohamud moved to the U.S. from Somalia as a young boy. Former neighbors speak highly of the suspect and his family.
He graduated from high school in Beaverton and dropped out of Oregon State University in October.
Meanwhile, the FBI is offering a $10,000 reward for information leading to the arsonist who set a mosque on fire where Mohamud used to worship. The mosque is located in Corvallis, which is a college town about 75 miles southwest of Portland.
Sunday's fire caused moderate damage and no one was injured.
Authorities have stepped up security at mosques in the area because Muslim leaders expressed fears of retribution.
The Associated Press Contributed to this report.