The four men arrested Wednesday night in what the authorities said was a plot to bomb two synagogues in the Bronx and shoot down military planes at an Air National Guard base in Newburgh, N.Y. were petty criminals who appeared to be acting alone, not in concert with any terrorist organization, the New York City police commissioner said Thursday.

The men were arrested in an elaborate sting operation at around 9 p.m. on Wednesday after planting what they believed to be bombs in cars outside the Riverdale Temple, a Reform synagogue, and the nearby Riverdale Jewish Center, an Orthodox synagogue. Once the explosives were planted, the men planned to drive to the National Guard base to shoot down military aircraft with a Stinger surface-to-air missile while detonating the bombs with a remote device.

The men did not know that the bombs, obtained with the help of an informant for the Federal Bureau of Investigation, were fake, and that the missile was incapable of being fired.

In a news conference at the Riverdale Jewish Center, one of the two synagogues said to be the targets of the plot, the commissioner, Raymond W. Kelly, offered new details about the four defendants  James Cromitie, David Williams, Onta Williams and Laguerre Payen  three of whom were arraigned in Federal District Court in White Plains, N.Y., on Thursday.