Miami ? A seaplane crashed just off Miami Beach within sight of the city’s high-rises Monday, killing all 20 people aboard. Witnesses said the aircraft exploded in flames as it came down, and the FBI joined the investigation.

Amateur video obtained by CNN showed the main part of the plane slamming into the water followed by a flaming object trailing thick black smoke.

Scuba divers and rescuers in speedboats struggled to reach the victims, but as evening fell, they found no sign that anyone survived.

The Chalk’s Ocean Airways plane – a twin-engine Grumman G-73T Turbine Mallard – went down around 2:30 p.m. after taking off from Miami for the island of Bimini in the Bahamas.

The Coast Guard said 19 bodies were found. The 1940s-era plane was carrying two crew members and 18 passengers, including three infants, authorities said.

Because of the witness reports of an explosion, the FBI sent agents to assist in the investigation, but there was no immediate indication of terrorism or sabotage, said Judy Orihuela, spokeswoman for the FBI’s Miami field office.

“It’s too soon to say whether we are going to get involved,” Orihuela said. “We’re just going to check it out.”

Chalk’s is a small air carrier that is not required to conduct federal security screening of passengers and their luggage, said Dale Karlen, federal security director at Fort Lauderdale-Hollywood International Airport.

With many schools already closed in advance of the holidays and Christmas week traditionally one of the biggest times of the year for tourism, the beach was relatively busy, and dozens of beachgoers saw the plane go down. Dozens more gathered to watch the rescue.