This could have been a disaster. Thankfully, no one was hurt.

Federal authorities have joined the search for a suspect who planted a crude car bomb of propane, gasoline and fireworks in a smoking Nissan Pathfinder in the heart of Times Square on Saturday evening, prompting the evacuation of thousands of tourists and theatergoers on a warm and busy night.

Although the device had apparently started to detonate, there was no explosion, and early on Sunday the authorities were reviewing surveillance tapes and forensic evidence as they sought out a motive.

“We are very lucky,” Mayor Michael R. Bloomberg said at a 2:15 a.m. press conference. “We avoided what could have been a very deadly event.”

A large swath of Midtown — from 43rd Street to 48th Street, and from Sixth to Eighth Avenues — was closed for much of the evening after the Pathfinder was discovered just off Broadway on 45th Street. Several theaters and stores, as well as a portion of the Marriott Marquis Hotel, were evacuated. The streets and hotels have since reopened.

Federal authorities said the incident appeared to be an isolated one, and that there was no evidence of an ongoing threat to the city.

“We are treating it as if it could be a potential terrorist attack,” Janet Napolitano, the Homeland Security secretary, told CNN, one of several television appearances she made on Sunday morning.

The bomb itself was not a sophisticated device, Ms. Napolitano said on ABC’s “This Week.” She said there was no clear sense about how large the explosion might have been. “Right now, we have no evidence other than it is a one-off,” she said.

On Sunday, police and F.B.I. officials were also investigating a 911 call placed at around 4 .a.m. on Sunday, several officials said. The caller, who one official said sounded intelligent, admonished the 911 dispatcher not to interrupt him until he was finished and then said there would be a massive explosion soon and the car in Times Square was only a diversion.

The call came from a payphone at West 53rd Street and Seventh Avenue, which detectives have since dusted for fingerprints, the officials said.

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