One person has been killed and three others are missing after a boat carrying 20 people sank near Alcatraz, the former prison and tourist site in San Francisco Bay, US officials said late on Tuesday.
Helicopters, planes and rescue boats operated by local fire and police teams were working alongside US Coast Guard to find those who are missing.
Fire Chief Dean Crispen said rescuers first responded to a report of "a boat on fire, 600 yards (548 metres) off Alcatraz" Island around 3:35 pm, according to a short Instagram video posted by San Francisco's mayor.
When rescuers approached, the boat had capsized and efforts were made to resuscitate the person who died.
"They immediately initiated CPR, transported that patient to the shore. They were declared deceased," Crispen said.
The fire department initially reported two missing, but later updated the toll to three, citing witness statements.
San Francisco Fire Department Lieutenant Elias Mariano told the AFP news agency the search for those still missing will continue "through the night."
Rescue teams were using "thermal imaging, tide prediction and modelling to help direct search efforts," the department said.
The boat was a privately owned cabin cruiser motorboat from Stockton in California, Mariano said.
Authorities did not know if the boat was headed to Alcatraz, a major tourist attraction, where shuttered buildings of the famous former prison still stand.
At least one person was severely injured in the water when rescuers arrived, the Los Angeles Times reported.
Authorities confirmed the remaining 16 passengers were rescued.
