By N. Clark Judd

Four men accused of attempting to detonate bombs outside two Riverdale Jewish institutions are being held without bail after brief court appearances on Thursday.

The men were arrested on Wednesday night after allegedly trying to plant car bombs in front of two Riverdale synagogues on Independence Avenue.

No one at Riverdale Temple or the Riverdale Jewish Center was hurt and no property was damaged. The explosives, which the suspects allegedly believed to be C-4, were inert, Police Commissioner Ray Kelly said at a Thursday morning press conference. A cooperating witness working for the Federal Bureau of Investigation provided the C-4 to the alleged would-be bombers, along with an inoperable Stinger missile launcher they planned to use to shoot down planes at a New York Air National Guard base near their homes in Newburgh, N.Y., according to a criminal complaint.

Wednesday night’s events were the culmination of an investigation that stretches back to last June, according to the office of the U.S. Attorney for the Southern District of New York. At each stage, officials stressed, federal agents monitored their prey, allowing the four accused to further their alleged plot — but ensuring it could do no harm.

When asked how the suspects picked their targets, Mr. Kelly said the synagogues’ proximity to the Henry Hudson Parkway may have made them convenient for James Cromitie, 55, David Williams, 28, Onta Williams, 32, and Laguerre Payen, 27, the men accused of attempting the bombings.

The suspects, Mr. Kelly said, are believed to have met in prison. Each has a criminal record, but their criminal careers are largely comprised of petty crimes, he said.

During court proceedings Thursday, Assistant U.S. Attorney Eric Snyder said Mr. Payen had twice been indicted and twice pleaded guilty, though he did not say what crimes Mr. Payen admitted to committing.