Witnesses told emergency personnel that the twin-engine plane "went in nose first," leaving a debris field three-quarters of a mile long in Trinity Bay, Chambers County Sheriff Brian Hawthorne said.
"It's probably a crash that nobody would survive," he said, referring to the scene as "total devastation." Witnesses said they heard the plane's engines surging and that the craft turned sharply before falling into a nosedive, Hawthorne said.
Aerial footage shows emergency personnel walking along a spit of marshland flecked by debris that extends into the water.
The sheriff said recovering pieces of the plane, its black box containing flight data records and any remains of the people on board will be difficult in muddy marshland that extends to about 5 feet deep in the area. Air boats are needed to access the area.
The plane had departed from Miami and was likely only minutes away from landing at George Bush Intercontinental Airport in Houston.
The Federal Aviation Administration issued an alert after officials lost radar and radio contact with Atlas Air Flight 3591 when it was about 30 miles (48 kilometers) southeast of the airport, FAA spokesman Lynn Lunsford said.
The Coast Guard dispatched boats and at least one helicopter to assist in the search for survivors. A dive team with the Texas Department of Public Safety will be tasked with finding the black box, Hawthorne said.
Trinity Bay is just north of Galveston Bay and the Gulf of Mexico. FAA investigators are traveling to the scene as are authorities with the National Transportation Safety Board, which will lead the investigation.