A new research suggests that hands-free headsets and built-in phones are no safer than handheld phones while driving.
The data suggests it is the conversation that distracts drivers and not the phones.
The US National Highway Traffic Safety Administration was accused of concealing hundreds of pages of the research on the dangers of using cell phones while driving since 2003.
The documents were said to have come into light only when public advocates had to turn to court to get them.
Margaret Kwoka, with the Public Citizen advocacy group, questioned the reason for keeping the papers under cover.
"From looking at these docs now, it seems like nothing more than an agency who knew about a serious safety problem and didn't know the public to know that they weren't doing anything about it," CBS News quoted Kwoka as saying.
A former number two official at the Transportation Department said he felt the study "wasn't clear."
Meanwhile, safety advocates have filed a petition demanding a ban of built-in phones and interactive devices in a moving vehicle.