Transit systems warned to be vigilant for terrorists

Zazi and two other defendants have not been charged with any terrorism counts, only the relatively minor offense of lying to the government. But the case could grow to include more serious charges as the investigation proceeds. Zazi has publicly denied being involved in a terror plot, and defense lawyer Arthur Folsom dismissed as rumor any notion that his client played a crucial role.

Publicly, law enforcement officials have repeatedly said they are unaware of a specific time or target for any attacks. Privately, officials speaking on condition of anonymity because they were not authorized to discuss the case said investigators have worried most about the possible use of backpack bombs on New York City trains, similar to attacks carried out in London and Madrid.

The investigation was ongoing to determine Zazi’s role and how many others might have been involved. Backpacks and cellphones were seized last week from apartments in Queens where Zazi visited.

In a bulletin issued Friday, the FBI and Homeland Security Department warned that improvised explosive devices are the most common tactic to blow up railroads and other mass transit systems overseas. And they cited incidents in which bombs were made with peroxide.

In the bulletin, officials recommended that transit systems conduct random sweeps at terminals and stations and that law enforcement make random patrols and board some trains and buses.

New York’s transit agency said it was coordinating with a task force made up of FBI agents and New York police.

The Massachusetts State Police were notified of the federal warning yesterday and were put on a heightened level of alert, spokesman David Procopio said. “The information was passed along to make sure all troopers are mindful of the specific intelligence,’’ he said.

The New York task force feared Zazi might have been involved in a potential plot involving hydrogen peroxide-based explosives, according to two law enforcement officials, who spoke on condition of anonymity because they were not authorized to discuss the investigation.