The NTSB's Kitty Higgins said the pilot had told controllers he had struck birds.
Pilot Chesley Sullenberger and his co-pilot are being interviewed by investigators.
While relieved passengers have spoken to the media of their ordeal in the freezing waters, Captain Sullenberger is not able to speak publicly while the inquiry continues.
He was widely praised for his skill in bringing down the stricken plane after both engines failed when birds apparently flew into them.
New York Mayor Michael Bloomberg also honoured the rescuers who pulled all the passengers and crew to safety.
One person suffered two broken legs in the crash and paramedics treated 78 patients, most for minor injuries.
'Controlled chaos'
Passengers and eyewitnesses have been speaking to the media about their experiences.
"I heard an explosion, and I saw flames coming from the left wing, and I thought, 'this isn't good'," said passenger Dave Sanderson.
"Then it was just controlled chaos. People started running up the aisle. People were getting shoved out of the way."
Another passenger, Billy Campbell, described water rushing into the plane as flight attendants did "a wonderful job" of evacuating everyone.
"It's good to be alive today," said Martin Sosa, a father who had been travelling with two young children and his wife.
After crashing on the water, the plane was pulled rapidly down the river, until it was guided to a halt by tug boats against a pier.
The temperature was almost -7C (19F) and the current in the Hudson was running rapidly.
1 1526 local time (2026 GMT): Flight 1549 takes off from LaGuardia airport 2 1527 (2027 GMT): Pilot Chesley Sullenberger reports birds hitting engines 3 1528 (2028 GMT): Pilot told to land at Teterboro airfield 4 1531 (2031 GMT): Pilot ditches plane in Hudson River
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