Coast Guard Rear Adm. Joseph Castillo said that there was a chance of life because the missing crew members could have been wearing drysuits and were in excellent physical condition.
"We continue until there is no more hope. We don't ever want to suspend the case prematurely, when there may be someone out there," Castillo said. "But hope gets less every day. My hope today is not what it was yesterday."
Jennifer Wiegandt Seidman she said hopes her husband, Chief Petty Officer John Seidman, was wearing the thermal protective gear when he entered the chilly Pacific Ocean off the coast of San Diego, where water temperatures have hovered in the lower 60s.
Seidman is a flight engineer with a 23-year career in the Coast Guard.
"I don't want to let my mind go to thinking the worst," Seidman said from the couple's home in Carmichael, Calif. "John knows what he's doing, and he's fit and he's very smart. They're saying that they're still looking."
The Seidmans married in 2001 and Seidman, 43, is stepfather to her three children, aged 10, 12 and 13, she said.
"I don't want to talk about him like he's gone," she said, choking back tears.
That possibility, however, loomed large over the rescue operation Saturday as Coast Guard helicopters came and went from a landing area near a popular waterside path that teemed with joggers and bikers - nearly 48 hours after the aircraft crashed at 7:10 p.m. Thursday.
Six Coast Guard cutters, three Navy ships and multiple helicopters were searching 644 square miles of ocean, but rescuers were concentrating on a debris field 50 miles off the San Diego coast. Rescuers have found debris from both aircraft, but there was no sign of the crew members or their bodies.
Castillo told reporters at a Saturday afternoon news conference that the mission was still considered search and rescue, not search and recovery.
He said that rescue teams will evaluate the operation overnight and decide Sunday whether it would turn into a recovery operation.
Thursday's crash involved a Coast Guard C-130 with a seven-member crew and a two-person Marine Corps AH-1W Super Cobra that was flying in formation near the Navy's San Clemente Island, a site with training ranges for amphibious, air, surface and undersea warfare.
The Coast Guard airplane was itself carrying out a search for a missing boatman, 50-year-old David Jines. Jines left Avalon Harbor on Santa Catalina Island Tuesday aboard a 12-foot motorized skiff. A friend reported Jines missing the next day. Jines was still missing as of Saturday afternoon.
The Marine helicopter was one of two Cobras escorting transport aircraft with Marines aboard en route to a nighttime training exercise on San Clemente Island.
All seven aboard the Coast Guard plane are stationed at the Coast Guard Air Station in Sacramento, Calif., where their aircraft was based.
The aircraft commander, Lt. Cmdr. Che Barnes, 35, is from Capay, Calif. His co-pilot, Lt. Adam Bryant, 28, is from Crewe, Va.
Bryant's mother, Nina Bryant, also of Crewe, said Saturday that all she had been told was that "they're searching and haven't found anyone yet, and they don't know whose fault it was." She said she is "hoping and praying" her son and the others will be found alive.
"You never know. Miracles happen," she said.
The missing crew members from the Marine helicopter are Maj. Samuel Leigh, 35, of Belgrade, Maine, and 1st Lt. Thomas Claiborne, 26, of Parker, Colo.
Leigh's father, David, said his son, who was not married, was based in San Diego and was focused on a military career "since age 3." Leigh went to Norwich University, a military school in Vermont, and joined the Marines right after graduation in 1996.
"He wasn't mechanically inclined, so we were particularly proud of him, because he had to master an awful lot," said David Leigh, who lives in Belgrade.
The family last spoke to their son Wednesday by phone. He told them he would be night-flying the following evening.
Claiborne's father, Kenneth, said from his Parker home the family "would like to remain in private right now."
The accident happened in airspace uncontrolled by the FAA and inside a so-called military warning area, which is at times open to civilian aircraft and at times closed for military use, Federal Aviation Administration spokesman Ian Gregor said. He did not know the status of the airspace at the time of the crash.
Minutes before the collision, the FAA told the C-130 pilot to begin communicating with military controllers at Naval Air Station North Island in San Diego Bay, but it was not known if the pilot did so, Gregor said.
Contributing were Daisy Nguyen in Los Angeles, Larry O'Dell in Richmond, Va., Dan Elliott in Denver, and Kathy McCormack in Concord, N.H.