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(CNN) -- Rescue teams continued scouring waters off Mexico's coast Thursday, searching for seven Americans on a fishing expedition missing since a boat capsized over the weekend.
None of the missing had been spotted by Thursday afternoon, U.S. Coast Guard Petty Officer 2nd Class Levi Read told CNN. But he said officials believe they could have survived, helped by warm water temperatures and calm seas.
"Conditions are probably about as good as we can expect," he said.
One American was killed when the boat with more than 40 people aboard sunk early Sunday in the Sea of Cortez, near Isla San Luis, Mexico, authorities said.
Tour boat sinks, Americans aboard
Sixteen of 19 Americans that have been rescued returned to the United States Thursday, the government of Mexico's Baja California state said. Three rescued Americans remained in Mexico as the search continued, the government said.
Relatives of the seven missing Americans are seeking donations to help continue rescue efforts after Mexican and U.S. teams end the official search.
The search has been extended indefinitely, Mexican officials said Thursday. It had originally been scheduled to stop around 2 a.m. Thursday, 96 hours after the boat capsized.
The situation has drawn attention from officials in California, the home state of many of the missing. On Wednesday a group of California lawmakers urged federal officials to encourage Mexican authorities to keep searching.
"With the warm weather conditions, we believe the missing family members may be able to survive for additional days at sea or on one of the many islands in the region," the lawmakers wrote in a letter to U.S. Secretary of State Hillary Clinton and U.S. Coast Guard Commandant Adm. Robert Papp.
Family spokesman Frederick Han -- whose 52-year-old father-in-law, Donald Lee, is among the missing -- said he was holding out hope.
"If we know there are still people out there who are going to make the effort to search for them, then we maintain the hope that they're going to be found," he said Wednesday.
Han said relatives have urged U.S. authorities to play a greater role, and have been getting the word out about the situation online at findourfathers.blogspot.com. Family members also launched a Facebook page "Find Our Fathers" and a PayPal account for donations to help with the search and rescue efforts.
U.S. and Mexican teams are part of the search, as the boat was carrying citizens of both nations.
A U.S. Coast Guard C-130 Hercules aircraft searched a 300-square-mile area near Isla San Luis Thursday. The plane will search the area again Friday, the Coast Guard said.
The Coast Guard said in a statement Thursday that it had searched 1,400 square miles of ocean and land since the Mexican Navy requested assistance in the operation.
U.S. authorities identified the American tourist who died as Leslie Yee.
In addition to Lee, the missing also include Albert Mein, Russell Bautista, Mark Dorland, Brian Wong, Gene J. Leong and Shawn Chaddock.
U.S. and Mexican authorities said there were 43 people on the boat when it sank. The U.S. Coast Guard originally said 44 people were onboard, but changed their number after learning that one person listed on the manifest had never boarded the boat.
CNN's Catherine E. Shoichet and Greg Morrison contributed to this report.
