PORTLAND, Ore. -- A 19-year-old Somali-American man has pleaded not guilty to an alleged plot to blow up a car bomb at an Oregon Christmas tree lighting ceremony in Portland.

Mohamed Osman Mohamud entered the plea Monday in federal court in Portland.

An indictment filed earlier in the day charged him with attempted use of a weapon of mass destruction.

Mohamud was arrested Friday evening near the crowded Pioneer Courthouse Square in downtown Portland, amid an FBI sting operation that followed months of investigation.

A judge set a tentative trial date for Feb. 1.

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The indictment and plea come as Attorney General Eric Holder defended federal agents's action in the case. The FBI set up a sting operation to investigate Mohamud after receiving a tip. Holder rejected the suggestion that Mohamud was a victim of illegal entrapment by the FBI.

Once the undercover operation began, the suspect "chose at every step to continue" with the bombing plot, said Holder.

"He was told that children -- children -- were potentially going to be harmed," the attorney general added.

Also Monday, information surfaced about a 2009 investigation of Mohamud on rape allegations, but he was never charged.

The Benton County District Attorney's Office didn't find a reason to charge Mohamud with a crime after a woman whose name was redacted in the documents filed a sexual assault complaint against him.

Mohamud had sex with the woman in an Oregon State dorm room on Oct. 31, 2009.

An analysis of the woman's urine failed to show any controlled substances or common pharmaceuticals.

Mohamud says in the documents he and the girl were "a little tipsy" when they left a fraternity party and returned to his dorm room.