Two policemen - an undercover cop buying cocaine and a uniformed patrolman on a routine call in a drug-infested area - were shot to death in upper Manhattan and police Wednesday searched for suspects in the unrelated slayings.

It was apparently the first time in the city's history that two officers have died in separate slayings on the same night, police said.Helicopters thundered overhead as shotgun-toting officers in flak jackets swarmed over the Washington Heights and Upper West Side neighborhoods where the officers were slain hours apart Tuesday.

Investigators knocked on doors and questioned residents as other officers used dogs to scour alleys and the northwestern tip of Central Park.

In the first shooting, on 105th Street near Central Park, Officer Christopher Hoban, 26, was shot in the forehead and side with a .357 Magnum in a frantic struggle with three suspected drug dealers after they recognized him as an undercover officer during a "buy-and-bust" operation, police said.

One of the suspects, tentatively identified as Manuel DeLarosa, 23, was killed in the scuffle and brief gunbattle, another unidentified suspect was arrested and the third escaped, police said.

Police recovered two weapons, including Hoban's, but failed to find the .357 Magnum they believed was used to kill the officer.

Hoban, a four-year veteran from Brooklyn who joined the Manhattan North Narcotics Unit in February, died at St. Luke's-Roosevelt Hospital Center about 20 minutes after the 7:10 p.m. shooting, police said.

About 2 1/2 hours later, Officer Michael Buczek, 24, of Suffern, N.Y., was shot twice in the chest on Broadway near 161st Street as he and his partner responded to a report of a sick woman.

The two officers had spotted two men acting strangely and ordered them to halt, police said. Instead, the pair ran. Buczek's partner captured one man but the other turned and shot Buczek twice before fleeing.

Buczek was in uniform but had failed to don his bullet-proof vest before going on patrol. The two-year veteran died at Columbia Presbyterian Medical Center about 10 p.m., about 15 minutes after being shot, police said.

Police Commissioner Benjamin Ward immediately ordered all officers to wear the protective vests, which had long been optional.