The search for the gun used to kill NYPD Officer Randolph Holder is a painstaking, low-tech operation in which police divers literally must scour the East River floor by hand for the weapon. As of late Friday no gun had been recovered.

Hampered by silt that can cut visibility to almost zero, NYPD divers are guided by a 75-foot rope placed on the river bottom from which they extend one hand in semicircular sweeps as they move along the line, police spokesman Stephen Davis told Newsday Friday. Cops call this kind of search "tactile." After one area is searched, the bottom rope is moved for the next sweep.

Because of the strength of tides, cops can effectively work only during three, 90-minute slack periods on the bottom off 123rd Street and the FDR Promenade, Davis said. That means they are able to function effectively in the river for about three hours a day.

"I would much rather look for a needle in a haystack then a gun on the bottom of the river," Davis said.

Police believe murder suspect Tyrone Howard tossed the gun he allegedly used to slay Holder on Tuesday night in the East River around 123rd Street. A clip from a .40-caliber was found in the water near that spot after two men fishing from the shore saw a man running north and then toss something into the river. Because of the two witnesses, a police boat was able to shine a spotlight on the spot in the river where they heard a large object hit, hastening recovery of the clip, according to Davis.

Though police have metal detectors, the divers -- working on the bottom with teams of two and one person above in a raft -- say they are useless in this kind of river that can hide all sorts of objects. In the search for the gun so far, cops have found tire rims, auto alternators and an old .22-caliber pistol that was rusted and apparently in the water for years, Davis said. An additional find Thursday was an old U.S. military ammunition belt containing five empty rifle cartridges.

Divers also found a 9-millimeter round, thought to be one of seven fired by Holder's partner Omar Wallach while pursuing Howard, who was struck once in the leg. Also found was one shell casing of the same caliber, according to Davis.

Because the gun appears to have been separated from the clip, it is possible that the river current may have taken it further away, Davis said.

The divers are being as methodical as possible, Davis said, because if they miss an area as small as six inches square the gun could be missed and never found. Visibility is said to be between 10 to 12 inches because of river silt.

NYPD divers will be working through the weekend if necessary, waiting for the slack tides so they can resume their search.