JUST IN: Radio transmission to the Black Hawk helicopter...

JUST IN: Radio transmission to the Black Hawk helicopter may have been "interrupted" moments before it collided with the American Airlines flight carrying over 60 people.

NTSB chair Jennifer Homendy says there is conflicting information regarding the Black Hawk's altitude.

The National Transportation Safety Board says a key transmission from the air traffic control tower may not have been heard by the helicopter pilots.

"At 8:47:42 - or 17 seconds before impact - a radio transmission from the tower was audible on both CVRs directing the Black Hawk to pass behind the CRJ."

"The instructions from ATC were to pass behind the CRJ. That transmission was interrupted, it was stepped on..."

"CVR data from the Black Hawk indicated that the portion of the transmission that stated 'pass behind the' may not have been received by the Black Hawk crew."

Homendy said the helicopter mic key may have been pressed at the time of the transmission.

The agency is also looking into "conflicting information" regarding the Black Hawk's altitude, suggesting the Black Hawk may have had a malfunctioning altimeter.