The police commissioner, Raymond Kelly, said yesterday that a sketch of Grand Central Terminal discovered on the computer hard drive of a man suspected of involvement in the Madrid train bombings did not constitute an imminent or substantial threat to New Yorkers.

The schematic of Grand Central is “an amateurish,” partly hand-drawn document that is only one page and features the train station’s cavernous sanctuary along with an adjoining room, Mr. Kelly said. The commissioner said he first viewed the drawing after it was found on the hard drive of a laptop computer seized last November.

Mr. Kelly also reported that the hard drive contained another basic schematic of a New York building, which he declined to identify by name but described as a mixed-use site that was “not a significant or prominent structure.”

Unlike schematics of potential terrorist targets in New York that have been uncovered recently by counter terror efforts of the New York Police Department, such as the Citigroup Center and the New York Stock Exchange, the rudimentary schematics of Grand Central and the unnamed site lacked the specificity and sophistication to raise the terror-alert level, Mr. Kelly said.

In the cases of the Citigroup building and the Stock Exchange, the police uncovered evidence that terrorists had been casing the buildings and documenting the patterns of security details, as well as doing “reconnaissance” work. In the sketches obtained after the Madrid train bombings, there was no evidence of anything more than a picture of two buildings, coupled with photographs, Mr. Kelly said.

“It’s not an operational plan. It’s not something that would indicate an immediate threat,” Mr. Kelly said of the find from Spain.

Nevertheless, he said, the police and transit security presence at Grand Central was increased at the time, as well as at Pennsylvania Station and other transportation hubs.

Mayor Bloomberg, too, said yesterday that the FBI and the Police Department had been aware of the Grand Central sketch.

“We’ve known about the data on this computer for a long time,” Mr. Bloomberg said during an interview on the WBLS radio show. “We’ve taken the appropriate steps … to beef up security at all the major transportation hubs – train stations and airports and bus stations, places where you say if a terrorist wanted to attack, they would.”

The sketch was found in the home of a Syrian, Mouhannad Almallah Dabas, who was arrested at Madrid last March 24, less than two weeks after the deadly train bombings that killed 191 people. Mr. Dabas was later released but is still considered a suspect, according to the Spanish-language daily el Mundo of New York, which first reported details of the Grand Central Terminal sketch.

In other statements yesterday, the Police Department’s commissioner for counterterrorism, Michael Sheehan, told reporters during an Interpol conference on bioterrorism in France that the NYPD is “very concerned” that Al Qaeda is trying to acquire chemical, biological, or radiological weapons.

Air detectors have been installed at many New York locations, indoors and outdoors, to take air samples, and the samples are checked by scientists daily for pathogens, Mr. Sheehan told the Associated Press, noting that the project was still in the testing phase.

“We don’t have any information that, at this time, they have that capability, but we do know they’re trying to get it,” Mr. Sheehan said of Al Qaeda. He declined to provide specifics.