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Naiz Khan says he didn't know of any connections his friend Najibullah Zazi could have had to al-Qaida.

An official tells The Associated Press that the FBI had put Zazi under surveillance in connection with a suspected plot to make homemade bombs. Zazi tells the AP that he knows he's under investigation and that he has no ties to terrorism.

Meanwhile, federal officials Tuesday urged local police to keep an eye out for materials that could be used to make homemade explosives.

In a warning obtained by the AP, the FBI and Department of Homeland Security list indicators that could tip police off including people with burn marks on their hands, face or arms, foul odors and large industrial fans or multiple window fans.

"You can rest assured, we take the security of this city seriously. And any time there's any evidence or suspicion or allegation, we investigate to the utmost and hopefully it'll turn out there was nothing that's the best outcome here, that it was a false alarm. But I'd rather have a lot of false alarms than miss a real problem."

Federal investigators armed with search warrants raided at least three homes in Flushing, Queens Monday morning.

So far, police have not announced any arrests.

One intelligence official called the threat very real, and emphasized its urgency.