As many as 24 people may be under observation in a suspected cross-country terror plot, as police in New York City stepped up patrols and increased random searches on mass transit Wednesday.
Reports surfaced of up to two dozen more people under scrutiny who may have knowledge of or involvement in a suspected terrorism scheme that led to the arrest of a Denver shuttle driver and two others. Federal and local officials declined comment.
Amid media reports that some public storage centers in the region were being raided for a second day in the New York-Colorado investigation, the FBI told FOX News that there may be "consensual searches" under way but they were not "raids."
Though one unidentified senior law enforcement source told ABC News, "I've never been so worried," and unnamed officials said they're afraid plots against New York City are "still alive," authorities speaking publicly sounded less alarmed.
Police in New York and Washington, D.C. downplayed the threats to their cities and any spike in security. They echoed federal authorities, saying there were no known or specific threats to the sites mentioned in federal bulletins issued earlier this week.
The FBI and the Department of Homeland Security bulletins warned law enforcement officials around the country to watch mass transit, stadiums, warehouses with rentable storage units and hotels for any unusual activity. No new search warrants have been filed in recent days, the FBI told FOX.
The feds circulated the internal alerts in the midst of the investigation into a suspected New York City-Colorado terror plot in which three people — airport shuttle driver Najibullah Zazi, his father and an imam — are in custody for allegedly lying to the government.
The three are due in court Thursday to face the charges.
The possible Al Qaeda-linked scheme may have included plans to detonate bombs stuffed into backpacks, much like the subway attacks carried out in Madrid and London, authorities say.
New York's Metropolitan Transportation Authority confirmed it was beefing up security measures at heavily traveled spots.
The NYPD and the FBI issued a vague joint statement Wednesday saying they were working together on counterterrorism efforts.
"The FBI and the NYPD work ... side by side in task forces on a daily basis," said New York Police Commissioner Ray Kelly and Joseph Demarest, a director at the FBI's New York division. "This collaboration is an essential part of what helps to protect New York City from another terrorist attack."
Meanwhile, police acting without the FBI's knowledge may have inadvertently helped blow the surveillance of 24-year-old Zazi, who is the focus of the investigation.
A criminal complaint suggests that NYPD detectives may have compromised the bomb plot probe at a sensitive stage by questioning the imam, who preaches at a mosque in New York City, about Zazi.
"They came to ask me about your characters," Ahmad Wais Afzali told Zazi in a secretly recorded Sept. 11 telephone conversation. "They asked me about you guys."
At least one of those NYPD investigators, referred to in the recently unsealed criminal complaint, works for a division that operates independently from an FBI-run terrorism task force.
The complaint also suggests investigators may have tipped off Zazi by towing and searching a rental car he was using on a New York City trip that heightened fears of an attack.
The maneuver, authorities say, produced evidence of bomb-making instructions retrieved from a hard drive on Zazi's laptop.
But it also apparently didn't get by the suspect: In the phone conversation with Afzali, Zazi said the car's disappearance convinced him he was being watched.
NYPD and FBI officials have denied that the potential missteps forced their hand in a series of high-profile raids last week, prompted Zazi to abort his New York visit and caused friction between the two agencies — which work together through the Joint Terrorism Task Force.
Asked Tuesday if he had any concerns about the handling of Afzali, the NYPD's Kelly declined comment on the investigation beyond what was in court papers, saying the probe was classified.
Police spokesman Paul Browne insisted the NYPD and the FBI "worked closely and successfully in this case and in scores of others." He, too, declined further comment.
Zazi, his 53-year-old father Mohammed Wali Zaziand and 37-year-old Afzali were arrested over the weekend and accused of lying to the FBI, but weren't charged with terrorism. They have denied the charges. The scale and scope of the plot remains unclear.
Law enforcement officials said Zazi may have been plotting with others to detonate backpack bombs on New York trains in a scheme similar to the attacks on the London subway and Madrid's rail system in the last few years. Backpacks and cell phones were seized in raids on apartments Zazi visited in New York.
In a statement, the FBI and the Department of Homeland Security said that while the agencies "have no information regarding the timing, location or target of any planned attack, we believe it is prudent to raise the security awareness of our local law enforcement partners regarding the targets and tactics of previous terrorist activity."
Afzali's attorney, Ron Kuby, has said his client had a history of giving police information as a community liaison and Muslim religious leader in his Queens neighborhood. Kuby claimed Afzali was doing their bidding by talking to Zazi and finding out what he was up to.
"My client is being blamed for an investigation botched by the authorities," Kuby said Tuesday. "It's much easier to blame some obscure Afghan imam."
FOX News' David Lee Miller, Catherine Donaldson-Evans and The Associated Press contributed to this report.