More than 1,000 people died over two years from an illegal version of the painkiller fentanyl, the government reported Thursday in its first national tally of those deaths.

The spike of overdoses seems to have ended, health officials said, pointing to law enforcement's shutdown of a fentanyl operation in Mexico in 2006.

The wave of fentanyl overdoses first came to light in Chicago in 2005, and by 2006 more clusters were identified in Philadelphia, Detroit and other cities.

Hundreds of deaths from the drug were gradually reported, often episodically in local newspapers. Thursday's report from the Centers for Disease Control and Prevention puts the toll at 1,013 deaths from early April 2005 through late March 2007.

"This was really an epidemic," said Dr. Steven Marcus, the executive director of New Jersey's poison control center and a co-author of the new report.

Some deaths from illegal fentanyl still occur, but the worst of the outbreak seems to have ended after authorities shut down a fentanyl-making operation in Toluca, Mexico, in May 2006, said Dr. T. Stephen Jones, the study's lead author.

"It almost disappeared entirely. The shutting down of the Toluca facility was probably a major factor," said Jones, a consultant retired from the CDC.

The new report is being published this week in a CDC's Morbidity and Mortality Weekly Report.

Fentanyl is a prescription painkiller, often prescribed for cancer patients and administered through a patch. But it also is a powerful, euphoria-inducing narcotic, 30 to 50 times more potent than heroin.

Illegally made versions of the drug are sold as a powder, often mixed with cocaine or heroin, and sometimes used as a heroin replacement. It's possible some heroin addicts are unaware fentanyl is part of their injection, some experts say.

Smaller outbreaks of fentanyl-associated deaths in addicts have been reported before, including the "China White" outbreak of the 1980s, famed for being so deadly that drug users dropped dead with needles still in their arms.

The latest outbreak was first noted in Chicago. Patients who recovered from overdoses said they had been given free heroin in orange and pink plastic bags by new drug dealers trying to attract more customers.

The Chicago cases are summarized in the July issue of Clinical Toxicology.

It wasn't until a cluster of overdoses seen in Camden, N.J., emergency rooms in April 2006 that federal officials were notified of the problem, by Marcus.

The resulting investigation was unusual, because health officials have been reluctant to spend time and energy investigaing deaths related to illicit drugs, Marcus said.

"The response when I deal with public health officials is; 'Drug abuse is a dangerous habit, and drug abusers know it's a dangerous habit, so why are we making a big deal out of it?'" he said.

The report distinguished deaths due to illegally made fentanyl from those due to illicit use of the pharmaceutical product. Medical examiners cannot tell the difference from what's seen in an autopsy, so investigators relied on drugs found at the scene and other information to separate the two.

Also, the investigators did not count cases in every city. The tally covers only two states — New Jersey and Delaware — and the cities of Chicago, Detroit, Philadelphia and St. Louis.

"It's an incomplete picture," Jones said.

National health statistics show the death rate from unintentional drug poisonings — most of them illicit drug overdoses — roughly doubled from 1999 to 2005.