Updated Jan. 7 at 3:50 p.m.: Revised to include additional details.

Officials say the pilot of a small Texas-bound aircraft that diverted from its original path Wednesday and headed into the Gulf of Mexico may have suffered from a lack of oxygen.

Bill Kinsinger (Best Fur Friends Rescue)

The pilot was identified Thursday as 55-year-old Bill Kinsinger of Edmond, Okla., an anesthesiologist who volunteered to fly rescued animals.

The Cirrus SR22T left Wiley Post Airport in Oklahoma City around 2:15 p.m. The pilot had filed a flight plan to land in Georgetown but did not, Federal Aviation Administration spokesman Lynn Lunsford said.

The North American Aerospace Defense Command sent two F-16 fighter jets from Houston to make contact with the plane after its pilot stopped responding to instructions from air-traffic control, NORAD spokesman Michael Kucharek said.

The jets dropped flares and flew in front of the plane to get the pilot's attention, Kucharek said, but the pilot -- the only person aboard the plane -- appeared to be unresponsive. They were later replaced by two F-15 jets from New Orleans that followed the plane until they neared Mexican air space, but returned to base because of the proximity and darkness.

The aircraft was last observed on the radar 219 miles northwest of Cancún, flying at 15,000 feet, Lunsford said.

The Cirrus SR22T was travelling from Oklahoma City to Georgetown but veered toward the Gulf of Mexico. (FlightAware)

U.S. Coast Guard crews were still actively searching for Kinsinger on Sunday.

Kinsinger, 55, a volunteer with the group Pilots N Paws, was flying to Texas to pick up Masaru, a disabled husky that was bound for a foster home in Oklahoma. On Friday, several pilots flew from Texas to Oklahoma with the husky to complete Kinsinger's mission.

Flight coordinator Monica Marshall was tracking Kinsinger's progress with the radar showed he went off course.

"He was the type of guy that went above and beyond," Marshall said. "If someone bailed on an assignment then he would just stretch and do it for you."

Best Fur Friends Rescue in Fort Worth said on Facebook that Kinsinger was passionate about giving dogs another chance at life.

"We are just devastated and are sending the most heartfelt prayers to his family and all those who love him," the group said. "One thing we know, Bill earned his angel wings a thousand times over."

Officials with the Coast Guard said Kinsinger may have suffered from hypoxia, a condition where the brain is deprived of oxygen. Hypoxia can cause confusion, nausea and hallucinations, and can be fatal if untreated.

The FAA requires pilots to use supplemental oxygen if flying above 14,000 feet; Kinsinger had filed to fly at 19,000 feet.

The Associated Press contributed to this report.