A new report by a public safety group throws into question the ability of police and firefighters to locate people through their cell phones when they dial 911 in an emergency.

The study is believed to be the first independent evaluation of wireless location technology and sends a clear message: Do not assume rescuers will know where you are if you call 911 from a cell phone.

The report was commissioned by the Association of Public Safety Communications Officials International, a group that has long been concerned about the limitations of the technology and the public’s unrealistic expectations of what it can deliver.