BY MIDWEEK, when DNA results from a third Kensington strangulation victim come back, police expect to know if Philadelphia has a serial killer on its streets. For now, cops and experts say they see disturbing similarities in three recent strangulations and three other chokings.

The FBI defines serial murder as "the unlawful killing of two or more victims by the same offender(s), in separate events," and federal law defines serial killing as "a series of three or more killings . . . having common characteristics," according to the FBI's website.

Police have linked the strangulations of Elaine Goldberg on Nov. 3 and Nicole Piacentini on Nov. 13 to the same perpetrator through DNA evidence. Authorities are awaiting DNA results from Casey Mahoney, 27, of East Stroudsburg, Pa., who was found strangled Wednesday.

All three bodies were found partially clothed and sexually assaulted in abandoned Kensington lots. Piacentini and Mahoney were positioned facedown, police said.

"Obviously, we have two," Lt. Ray Evers, police spokesman, said yesterday. "If the third victim's DNA matches, and everything is leading to that because everything else is matching - the motive, victim, posing, how it's done - then by pure definition we have an issue on our hands."

Three other women since early October claim to have survived being assaulted and choked into unconsciousness in Kensington.

James Alan Fox, a criminologist at Northeastern University and author of six books on murder, including his most recent, Extreme Killing, said serial killing tends to be about power, control and sexual domination. The act of murder, for a serial killer, is intimate, he said.

Fox said the so-called Kensington Strangler fits the profile of a serial killer, partly because it's more typical for a serial killer to carry out the crime with his own hands than with a gun.

"Guns are not very common because guns distance them from their victim," Fox told the Daily News yesterday. "If the purpose is to eliminate, then a gun is the best means, but if the purpose is to feel the victim's last breath leave their throat, then it's critical you have contact."

Strangulation was the method used by Philadelphia serial killer Harrison "Marty" Graham, who was convicted in 1988 of strangling seven women and hiding their bodies in his North Philly apartment. Graham is serving a life sentence at a state prison in Coal Township, Northumberland County.

The most typical victim for a serial killer is a prostitute, Fox said. Police have stopped short of saying that any of the Kensington Strangler's victims were prostitutes, but the crimes happened in an area known for drug trafficking and prostitution.

Fox said that prostitutes are easy targets because they voluntarily go with strangers, and because a sexual undertone already exists and a serial killer can select victims based on his sexual fantasy.

"Just like men and women can have preference in a sexual partner, so can serial killers when it comes to their victims," Fox said.

In fact, a prostitute who escaped Philadelphia serial killer Gary Heidnik's car in 1987 led police to his "House of Horrors," where three women were chained in the basement, raped and forced to eat the remains of one of two other women he killed. Heidnik was convicted of two counts of murder in 1988, and died by lethal injection in 1999, the last person killed on Pennsylvania's death row.

Fox said it is unusual that the Kensington Strangler - described as a medium-built black man with long sideburns - apparently has singled out white women in their 20s and early 30s.

"If this is sexually motivated, there is a tendency to seek out those of the same race," he said.

He said that at most, 1 percent of homicide victims each year in the U.S. are murdered by serial killers.

"The fact that 1 percent of murders can be traced to a handful of people is alarming, though," he said. "These men, in some respect, are killing machines."

Evers said patrol units are on the streets of Kensington, serving bench warrants, reaching out to informants and locking up prostitutes and johns who continue their high-risk behavior.

On the weekend of Dec. 11 and 12, police locked up 20 johns and 14 prostitutes, Evers said. Any john who matched the description of the Kensington Strangler also was swabbed for DNA, he said.

Meanwhile, police are hoping that someone will recognize the man believed to be the Strangler from videos and composite sketches and will speak up, Evers said.

"It's coming down to the fact that we need the public to step forward and say, 'Hey I know this guy,' " he said. "We just need that one break, and it's really going to come from the public."

If the DNA comes back from Mahoney as a positive match to the other strangulation victims, Evers said, the community will ratchet up its vigilance. But he cautioned against connecting the cases until definitive evidence is returned.

"What if this DNA doesn't come back?" he said. "Now we have two guys doing the same stuff. Now we're looking for two guys."

Anyone with information on the cases is urged to call police at 215-686-TIPS.