Feds Still on Guard for NYC Terror Plot

Last Updated at 2:33 p.m. Eastern

The alleged terror plot aimed at New York City that has led to the arrests of three men may be only partially disrupted, law enforcement officials cautioned Monday.

Authorities told CBS News producer Pat Milton that they may not know the full scope of the alleged conspiracy and could not rule out the continuation of the plans by other conspirators in New York or other cities.

New York is widely assumed to be the target of the potential attack, but other locations could not be ruled out.

Authorities say Najibullah Zazi, a 24-year-old Colorado resident born in Afghanistan, is a central figure in the plot. Zazi along with his father, Mohammed Wali Zazi, in Denver Saturday.

Ahmad Wais Afzali, a 37-year-old imam in Queens, NY was also arrested.

The three are charged with lying to federal investigators and are expected to appear in court Monday.

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

Backpacks and cell phones were taken from apartments in the Queens raids last week.

Law enforcement officials told CBS News that they've identified a "core group" of a dozen individuals in New York and Denver who were either aware of or had contact with the three suspects.

Those people are aware that they are under suspicion and some, but not all, have been interviewed by authorities.

A joint FBI-New York Police Department task force feared Zazi may have been involved in a potential plot involving hydrogen peroxide-based explosives like those cited in an intelligence warning issued last week, according to two law enforcement officials, who spoke on condition of anonymity because they were not authorized to speak about the investigation.

On Monday, federal officials reminded law enforcement across the country that rail and transit systems can be vulnerable to terrorist attacks.

In a joint assessment, the FBI and Homeland Security Department warned that improvised explosive devices are the most common tactic to blow up mass transit and rail systems overseas. And they noted incidents where homemade bombs made with various types of peroxide.

In the assessment, obtained by The Associated Press, officials recommended that transit system security officials conduct random sweeps at terminals and stations and that law enforcement make random patrols and board some trains and buses.

Officials also told CBS News they are continuing to consider more substantial charges against the suspects in custody. Right now, authorities are using the charges of making false statements to the government as leverage in the hopes of getting additional information.