With DNA test results confirming that a third woman died at the hands of the so-called Kensington strangler, the attacker has officially been classified as a serial killer, police said Tuesday.

The third victim, 27-year-old Casey Mahoney, was found a week ago in woods above train tracks running along East Tusculum Street.

Because she had been strangled and her body positioned in a way similar to one of the other victims, detectives immediately suspected she had been raped and killed by the same man. The DNA confirmed their suspicions.

"We now know that this predator is responsible for three murders in this area," said Homicide Capt. James Clark, whose unit is leading the task force investigating the killings.

"As a result of this being the third murder in a short period of time, in the same area, with the same type of victims, we do at this time consider it to be a serial killer," he said.

The body of the first victim, 21-year-old Elaine Goldberg, was found in a lot Nov. 3. The second, 35-year-old Nicole Piacentini, was found at an abandoned building Nov. 13. Mahoney was found last Wednesday.

None of the victims was from Kensington, a neighborhood teeming with drugs and prostitution, but all three had struggled with addiction. The victims were all sexually assaulted and strangled.

Police are cautioning that the man depicted in a composite sketch and surveillance video released to the public may not be the strangling suspect.

The composite was drawn based on the recollections of a woman who said she was choked unconscious in early October. The victim in that attack said she might have been sexually assaulted. She did not contact police until weeks later, and there is no DNA evidence to link that attack to the strangler.

"Don't get too caught up on the photograph," Police Commissioner Charles H. Ramsey said Tuesday.

Mayor Nutter also announced Tuesday that a $30,000 reward had been posted for information leading to the arrest and conviction of the assailant.

"We are serious about getting this psycho off the streets," Nutter said at a news conference in Kensington. "We will not tolerate this insanity."

The city posted $25,000, and the Citizens' Crime Commission of the Delaware Valley raised its previous reward to $5,000.

The city police union, Fraternal Order of Police Lodge 5, is offering an additional $5,000 for information leading to an arrest and a DNA match. Councilman Frank DiCicco has added $2,000 to that.

"Once he's brought in, if the DNA matches, you're paid," FOP president John McNesby said.

The news conference, held at the scene where Piacentini's body was found, was attended by city officials, police, FBI agents, and neighborhood residents.

"Our community is united in its efforts to get this dangerous individual off the streets," Nutter said. "We cannot allow this type of activity to go on in our great city."

Clark said police had been receiving tips nonstop, both before and after the reward was increased.

"We're asking anyone who has any information, even if they think it may not be linked to this, to contact the Homicide Unit and let us sort it out," he said. "We have detectives working day and night, following every lead."

Police have arrested more than 120 men and women in the neighborhood in recent weeks on prostitution-related charges and taken DNA swabs from 154 men, Deputy Commissioner William Blackburn said. Forty-five of those samples have been excluded from the case, he said.

The national CODIS database, containing thousands of DNA profiles of sex offenders across the country, has returned no matches to the killer's genetic material. That may indicate that he is young, Blackburn said, or that he has no significant criminal record.

Police also are concerned about vigilante actions gone awry.

On Monday, a rumor started that a Kensington resident, Triz Jeffries, was responsible for the stranglings - with fliers being circulated in the neighborhood with his name and address.

The flier also appeared on a Facebook page dedicated to spreading information and updates about the stranglings, and several Kensington residents said Jeffries' photo was forwarded to their cell phones.

Jeffries saw a crowd gathering outside his house Monday and became alarmed. He called police, who took him in for questioning and released him, saying he was not a suspect.

Police said that whoever started passing out Jeffries' name and address could face charges.

"We don't want vigilante action," District Attorney Seth Williams said. "Leave that to law enforcement."

Authorities urged people to call 911 if they see anything suspicious or if think they have spotted the man in the sketch, and cautioned residents against taking matters into their own hands. Anyone with information is asked to call 215-686-TIPS.

"This is not TV," Nutter said. "This is not the movies. This is a dangerous person. Don't play around with this."

Contact staff writer Troy Graham at 215-854-2730 or tgraham@phillynews.com.