The National Transportation Safety Board will be taking charge of the investigation.
Data from flight tracking services shows that the Boeing 767 entered a steep dive just before the crash.
It nosedived into the water near Trinity Bay, where Hawthorne said strong winds had made recovery efforts more hard by blowing water out of the bay, making it more hard to send certain boats to the area.
NTSB Chairman Robert Sumwalt said Saturday evening that preliminary information indicates "the aircraft was in what I would characterize as a normal descent, and at around 6,300 feet it began what I would characterize as a very, very rapid descent at that point". It was operating a flight for Amazon Air.
Police so far do not know what cargo the plane was carrying. "Our team continues to work closely with the NTSB, the FAA and local authorities on the ground in Houston".
In a statement, parent company Atlas Air Worldwide said: "We can confirm there were three people on board the aircraft".
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The jumbo jet had departed from Miami and was likely moments from landing at Bush Intercontinental Airport in Houston when witnesses said it crashed nose first into the bay.
The Sheriff's Office said deputies located the plane at the north end of Trinity Bay. The aircraft was part of the Amazon Prime Air Fleet and was traveling from Miami to Houston. "Our focus is on our friends and colleagues who were on that plane, and we are doing everything we can to support their families".
Hawthorne said the first things he saw in the water at the crash scene were things like bed sheets, women's clothing, and fiberglass.
Three passengers were on board but the FAA said no one survived. 'That's a negative, ' he said.
They also asked a Mesa Airlines pilot: "See if you can make ground contact". During its approach to George Bush, the 767 lost radio and radar contract with air traffic controllers Saturday as it crashed about 30 miles from the airport.
"No ground contact from here", the Mesa pilot said.