"I have never been so worried," said one senior law enforcement official with more than a dozen years of experience in counter-terrorism investigations.
In Washington, the FBI and the Department of Homeland Security issued a bulletin to law enforcement authorities urging vigilance at so-called "soft targets" including sports stadiums and luxury hotels.
ABCNews.com reported Monday that Zazi's computer contained information relating to New York area baseball and football stadiums and a video of Grand Central Terminal in New York.
Earlier, the FBI and Homeland Security had warned police about possible attacks on mass transit targets but said there was no evidence of any specific target or timing.
Law enforcement authorities tell ABCNews.com that more than 24 men in New York have been under watch at various times since Zazi's trip to the city on September 11.
One official said local and federal surveillance teams are "stretched thin" as authorities seek to track a group of young men allegedly recruited by Zazi following his return from an al Qaeda training camp in Pakistan in January.
Officials said some of the initial subjects had "washed out" but that others had emerged as "possible players."
Officials told ABCNews.com that Zazi had organized three distinct teams of four men each and that the investigation had led to more than ten others.
"This investigation is going forward aggressively," New York Police Commissioner Ray Kelly said Tuesday. "We are doing everything we can to protect the city."
Authorities cautioned there is always the possibility of the case "fizzling out," but for a tenth consecutive day, agents were attempting to find a garage or storage shed where they believe Zazi and others may have stored chemicals and bomb components.
Since a series of raids in New York and Denver, authorities say Zazi's alleged role as organizer and recruiter has come into sharper focus.
Zazi, who had lived in New York since he and his family arrived from Afghanistan in 1999, traveled to Peshawar, Pakistan in Aug. 2008.
According to an FBI affidavit filed in connection with his arrest, Zazi spent four and half months in Pakistan and received weapons and explosives training at an al Qaeda camp located near Peshawar.
Authorities tell ABCNews.com that several other men from the U.S. traveled with Zazi to Pakistan.
Upon his return to the U.S. on Jan. 15, Zazi moved to Denver and went to work at an airport shuttle van service.
Law enforcement authorities say he has been under FBI surveillance since then and that he traveled to New York at least one time before his September trip.
Authorities believe the earlier trip or trips to New York were designed to organize others he allegedly recruited for his network.