Pimentel was characterized as a "stoner" who was not a real danger to anybody other than himself, a federal source told the New York Post.
His mental faculties were also questioned, considering he once tried to circumcise himself, said another source.
Pimentel also seemed clueless about the gravity of his situation, suggesting that cops give him just a slap on the wrist for the crime and set him free, sources said. "Am I going to get a desk-appearance ticket?" he asked before insisting, "I'm not a terrorist."
Federal officials -- who sit on a joint terrorism task force with the NYPD -- were noticeably absent Sunday night from a news conference announcing Pimentel's arrest on state charges, but no federal ones.
But Police Commissioner Raymond Kelly said Monday, "There was no question in my mind we had to take this case down. This was an imminent threat."
Pimentel has pleaded not guilty and is being held without bail.
New York officials said they decided to arrest Pimentel because he had nearly finished assembling a pipe bomb built from match heads, Christmas lights, and an alarm clock. Law enforcement officials said they had become concerned the bomb components might pose a danger to neighbors in the apartment buildings where Pimentel and the informant lived. Pimentel, they said, planned to use the bombs against police officers, returning military veterans, and post offices.
His mom apologized Monday for her malcontent son -- and said he has to face "justice."
"I want to apologize to the City of New York," Carmen Rosa, a 57-year-old immigrant from the Dominican Republic, said. "I've been here since 1987, and I'm disappointed with what my son was doing. I love New York."
"I'm sorry for what my son did," she added. "I just want to say I love him. I didn't raise him that way -- he changed."
Rosa said that Pimentel, who is Muslim, first became interested in Islam after 9/11 and converted about five years ago.
"He liked religion. He liked being Muslim. He liked to read and write about what happens in the Muslim world," she said.