One person was killed, two were missing and 16 were rescued after a pontoon boat capsized and sank near San Francisco’s Alcatraz Island on Tuesday, authorities said.

Nineteen people were aboard the vessel at the time of the incident.

Initial reports indicated a fire had broken out on board, but San Francisco fire chief Dean Crispen said in a Tuesday evening press conference there was no indication of a blaze. It is still unclear what caused the boat to capsize.

First responders found a “severely injured” individual upon arriving at the vessel in the San Francisco Bay and initiated CPR. The person was brought to shore, where they were declared deceased.

Of the 16 people rescued, 13 were safe on shore, while three others were receiving medical treatment at a San Francisco hospital. Some of the passengers had impact injuries from falling in the water. A dog was also declared dead. All the passengers are believed to be adults.

The US coast guard confirmed search-and-rescue efforts were “ongoing” Tuesday afternoon.

“We are going to continue for hours to make sure that we find these two missing people if possible,” said Crispen.

Authorities have dispatched 11 boats and some helicopters to aid the efforts, as well as divers.

Crispen did not have an immediate figure on the size of the search area. He said if any individual was in the water, tides and the winds indicate they would be moving east.

“This is an all-hands-on-deck search and hopefully rescue,” San Francisco Mayor Daniel Lurie said.

Lurie also extended condolences to the family and friends of the deceased passenger.

When authorities arrived, the boat was leaking fuel, and exhaust was emitting from the still-operating motor, Crispen said. Just the top level of the boat was above water when authorities arrived. The boat has since sunk beneath the surface of the bay.

The vessel, described as a “pontoon pleasure boat” by San Francisco fire Lt Mariano Elias, was about 600 yards (548 meters) from the island when it capsized. It is believed to have set sail from St Francis Yacht Club, located in the city’s Marina district, Crispen said.

Live video from the scene showed a man and a woman wrapped in blankets and sitting near an ambulance.

Millions of visitors descend upon Alcatraz Island every year to see the eponymous, now-shuttered prison where some of the nation’s most infamous criminals were once incarcerated.