Najibullah Zazi, the airport shuttle driver who prosecutors say was planning an attack on commuter trains in New York City, was taken by federal marshals to the Metropolitan Detention Center in Brooklyn on Friday after being transported from Denver.

"He wasn't political and he didn't appear to be particularly fanatical in his Muslim beliefs," Richard Gross, a spokesman for ABC Transportation, said. "And he didn't try to impose his beliefs on anybody. So, he kept to himself pretty much and he never gave any outward signs of being connected with anybody."

Gross says Zazi was a hard worker and very quiet, but didn't seem like he had a lot of social skills.

"The best adjective to describe him is 'hardworking,'" Gross said. "He would work approximately 80 hours per week and he was known to his relatives as being somebody who was fairly stingy, and that's the best way to describe him."

According to Gross, Zazi had worked for ABC Transportation for six months, but before that he worked for another shuttle company at Denver International Airport.

When asked what kind of background check had been conducted on the terror suspect, Gross said Zazi's previous employer had required a vetting process conducted by the city and the Transportation Security Administration (TSA), which he says Zazi had been through on four different occasions. Gross says that check included a lengthy application process and a review of Zazi's green card application.

Gross did not say if ABC Transportation had done its own background check on Zazi.

The FBI, on the other hand, did find some red flags during its investigation of Zazi - a nine-page jpeg document containing handwritten bomb-making instructions was found on Zazi's laptop computer when his rental car was searched in New York City on Sept. 10.

According to an arrest affidavit, Zazi told authorities he must have inadvertently downloaded the file. However, the FBI says there were commonalities between the handwriting on the notes and samples Zazi submitted to investigators. Prosecutors have also alleged that Zazi received explosives training from al-Qaida in Pakistan.

Zazi's attorney, Arthur Folsom, says there's a key element missing from the prosecution's case against Zazi: explosives or the chemicals allegedly used to make them.

Investigators, however, say Zazi and three associates went to a number of beauty supply stores in Denver over the summer and used stolen credit cards to buy chemicals that could be used to make explosives like the ones that killed dozens in the transit bombings in London and Madrid.

Gross says Zazi's co-workers at ABC Transportation have viewed the situation with shock and surprise, and never saw any indicators of anything unusual.

"And that's what makes this so interesting is that there really wasn't much signs of him being involved with anything like this. He didn't discuss any type of anti-Americanism or anti-capitalism," Gross said. "He didn't discuss any of that with his co-workers, and he was most interested in working hard.

"His clientele seemed to appreciate his efforts. He didn't try to preach to anybody or to proselytize in any way," Gross continued. "And he was basically a very hardworking guy who really was about saving his money as much as possible."