Police are hunting for the source of an ecstasy pill that killed a 15-year-old Ajax girl last week, and are warning that a deadly batch of the drug may be circulating in the Greater Toronto Area.

The teenager, whose name has not been released, died in hospital of multiple organ failure on July 19.

Four days earlier, she had collapsed at a shopping mall and slipped into a coma after consuming a dose of ecstasy.

Durham Regional Police have launched a criminal investigation into her death, and are now seeking the source of the distinctively marked pill she took.

"The whole pill is yellow and, on the top or one side of the pill, would have a recessed stamp of a musical note, two eighth notes to be specific," said Durham police Det. Tom Andrews.

Police have not said whether they believe the victim died from an overdose, received a pill that was contaminated, or simply had a bad reaction to a standard dose.

Meanwhile, they have warned parents and teens to be on the lookout for pills similar to the one that led to the girl's death.

An autopsy has been performed, but the results have not yet been released.

Area experts say the teen's death confirms a worrying trend of teenage girls, rather than boys, experimenting with the drug in higher numbers.

"We're also seeing a trend in the increase of youth identifying ecstasy as a primary substance of choice," said Angela Kirby, who co-ordinates a drug treatment program at Durham Region's Pinewood Centre.

The greatest use of the drug, which is also known as methylenedioxymethamphetamine (MDMA), seems to be among youths aged 15 to 17, she added.