The police and health departments are investigating whether six deaths in Lower Manhattan during the past week were caused by a batch of tainted heroin.

Police Commissioner Raymond Kelly said officers are attempting to pinpoint the source of the heroin by interrogating anyone arrested recently for hard drug use or drug trafficking, interviewing friends and relatives of the deceased, expediting the lab tests from the six cases to determine whether a substance in addition to heroin may have been a factor in the deaths, and communicating with “confidential sources throughout the city.”

Precinct and housing police officers are alerting the public of the possibility of the bad batch of heroin, and the city health department has issued a health alert to area hospitals, which are asked to report similar drug cases immediately. It is also notifying methadone clinics, syringe exchange programs, and other programs that work with people who use drugs.

“In this case, I don’t believe there is such a thing as exercising too much caution,” Mr. Kelly said at a press conference yesterday.

On Wednesday, Kristopher Korkowski, 24, died either at or shortly after he was removed from 223 Avenue B in the East Village, police said. Two days later, Ivan Rivera, 25, was found dead on a landing at 238 E. 7th St. Also on Friday, two 18-year-old women, Mellie Carballo of Manhattan and Maria Pesantez of Queens, fell unconscious at an apartment at 484 E. Houston St. and were taken to Cabrini Hospital. Carballo was pronounced dead on arrival, and Pesantez died on Sunday.

On Saturday, the body of Charles Sicker, 37, was found on Pier 54 in the West Village. Yesterday, Anatoli Filistovich, 42, died either at or shortly after his removal from 260 Spring St., police said.

“Heroin, in and of itself, is deadly,” said the city’s commissioner of health and mental hygiene, Thomas Frieden. The health department estimates that on average, 900 people die in the city of drug overdoses each year. Of those, 700 are from opiates, including heroin.

A professor of security management in the department of law and police science at John Jay College of Criminal Justice, Robert McCrie, said he does not believe it was heroin alone that killed Carballo and Pesantez.

Pure heroin, he said, is too strong to be taken alone. It must be cut, or diluted, with other substances on its way to the user from the laboratory, through the distributors, to the street pusher.

Quinine and sugar, which can cause harmful reactions in users, are often used as adulterants because they give users a feeling of fullness, he said, but pushers are always experimenting with different types of substances. Talcum powder is also occasionally used, Mr. McCrie said, and it is sometimes mixed with milk sugar. The body often does not react well to talcum powder, he said. It may be an acute reaction to a new cutting agent or combination of cutting agents, likely aggravated by alcohol or other sedatives, that is killing heroin users.

Mr. McCrie said that in a straight drug overdose, the user goes into a stupor that lasts several hours before he or she dies. That normally allows enough time for the user to get medical attention. “Those women died awfully fast,” he said of the 18-year-olds, an NYU student and a Hunter College student, both graduates of a Catholic high school in the city, whose deaths have been the focus of intense attention by the city’s tabloids.

Police received a 911 call at 6 p.m. Friday, and Carballo was officially pronounced dead on her arrival at Cabrini Hospital at 6:22 p.m., according to police. “It’s likely that they had a systemic reaction, likely a reaction to drugs used for cutting,” Mr. McCrie said.

Dr. Frieden joined Mr. Kelly for the press conference yesterday afternoon at One Police Plaza. The health commissioner said that his department is investigating the six recent cases, and looking for any similar ones, to determine whether an “adulterant” is the linking factor in the cluster of deaths.

“This is a sharp experience for the heroin-user community,” said Mr. Mc-Crie. “They’re all aware that there are some untoward effects from using heroin,” he said, but the high-profile deaths of the two girls “came as a shock,” he said, for “those people that feel they’re really not taking much of a risk in using heroin.”

“It’s going to have a deterrent effect on drug abusers,” he said.

According to statistics from the federal Drug Enforcement Agency, statewide heroin seizures have risen in the past two years. In fiscal year 2003, the agency seized 24.5 kilograms of heroin. In 2004, it seized 168.3 kilograms, and already this fiscal year, which ends in October, it has seized at least 124 kilograms. Fifty of those kilos were seized by law enforcement agents yesterday, when they raided a Bronx based narcotics operation.

Police said it was “very possible” that the heroin would be tested to see if it contained adulterants that might have caused the deaths in Lower Manhattan.

As of 5 p.m. yesterday, Phoenix House, a well-known drug-rehabilitation organization, had received no warning from the city about unusually dangerous drugs, according to an employee of the organization, Justin Bernbach.