The evacuation of the U.S. Gulf Coast took a tragic turn Friday when a bus carrying nursing home patients fleeing Hurricane Rita caught fire on a gridlocked Texas highway, killing as many as 24 elderly evacuees, authorities said.

The National Hurricane Center downgraded Rita to a Category 3 storm on its 2 p.m. EDT advisory, saying the hurricane, now churning winds of 125 mph, was on a "slow weakening trend." The advisory said a "further slow weakening" was possible before expected landfall early Saturday.

"Rita is still expected to come ashore as a dangerous hurricane," the National Hurricane Center warned.

President Bush canceled a scheduled trip to San Antonio Friday afternoon, the White House said, after the staging area for a search-and-rescue operation that he had planned to inspect was relocated.

Bush called off the trip to his home state minutes before he was scheduled to depart. He had planned to fly to San Antonio to check up on preparations for the Rita relief effort. Afterwards, he was to go to Colorado Springs to visit the headquarters of the Northern Command at Peterson Air Force Base, where planning and staging for response and relief activities also is underway.

The president instead is flying straight to Colorado.

White House spokesman Scott McClellan said a shift in the storm's path prompted officials to relocate the search-and-rescue teams from San Antonio to a site closer to where Rita is expected to make landfall. "We didn't want to slow that down," McClellan said.

Earlier Friday, while visiting the Federal Emergency Management Agency headquarters in Washington, Bush said in response to a reporter's question that he did not want his visit to "interfere with the important work that will be going forward."

The bus fire south of Dallas came as Hurricane Rita barreled toward the Louisiana-Texas border, prompting one of the largest evacuations in U.S. history.

The bus carrying the elderly evacuees exploded into flames on Interstate 45 south of Dallas and closed the highway, a major escape route out of the area already bumper-to-bumper with motorists trying to leave ahead of the monster hurricane. The bus fire caused a huge back-up on the interstate.

The bus was one of two evacuating patients from the Brighton Gardens Nursing Home in Bellaire, an incorporated city in southwestern Houston. On board were 45 people, including 38 nursing home patients, six staff members and the driver, said Cindy Siegel, the mayor of Bellaire.

The fire occurred about 6:45 a.m. Friday after the bus pulled over on I-45 because of a flat tire or mechanical problem. As the problem was being addressed, there was an explosion on the bus, followed by a fire, Siegel said. Some patients on the bus had oxygen tanks to help them breathe, but it was not immediately clear what triggered the explosion and fire.

In a televised news conference, Siegel said she was informed that at least one surviving patient was taken to a hospital with injuries, but that the driver and the staff members were unharmed. There were other reports that several patients were taken to a hospital for treatment of smoke inhalation.

Siegel said other patients at the nursing home had been picked up by family members. Those who were evacuated by bus did not have anyone to pick them up, she said. The bus set out for Dallas Thursday evening but made slow progress because of huge traffic jams as evacuees headed north.

Brighton Gardens made "the right decision" to evacuate its patients and was following established procedures, Siegel said. She cited the example of New Orleans when special-needs patients were left behind in a nursing home and a hospital and perished after Hurricane Katrina caused devastating flooding.

Television footage showed the charred shell of the bus surrounded by police cars and ambulances. Emergency personnel tended to the injured on stretchers.

Dallas County Sheriff's Department spokesman Don Peritz said early indications were that the fire started because of mechanical problems, which in turn caused passengers' oxygen tanks to explode, the Associated Press reported. There were a series of explosions, he said.

"The driver did survive the accident," Peritz said. "It's my understanding he went back on the bus several times to try to evacuate people."

Traffic remained backed up for 50 to 100 miles with motorists trying to evacuate the Galveston and Houston areas of Texas. TV aerial pictures showed a line of cars extending as far as the eye could see. Airports were jammed with people trying to leave as well.

Texas Gov. Rick Perry (R) said at a late morning news conference that many motorists had run out of gas during the evacuation. Officials were making "bulk deliveries" of fuel by truck and helicopter directly to the highways, he said. Cars were also overheating in the long lines, television reports said.

The Texas National Guard refueled 260 commercial buses with 25,000 gallons of fuel as of this morning, a National Guard statement said.

Perry said he was waiting for information on the cause of the bus explosion.

"It's a great test . . . but we're going to get through this," he said of the approaching hurricane. "So be calm. Be strong. Say a prayer for Texas."

In Beaumont, Tex., National Guard and Army units used military transport planes to evacuate hundreds of invalids from hospitals, nursing homes and private residences Friday, Washington Post staff writer Doug Struck reported.

Evacuees were brought to the regional airport in Beaumont, carried on stretchers and loaded onto baggage carts, eight to a cart. They were then driven to the back of a C-5 Galaxy transport plane, where soldiers wearing gloves carried them into the cavernous hold for a flight to Dallas. Other aircraft were bound for San Antonio and other cities.

As the evacuation continued, intensifying wind, rain and waves gave coastal regions of Texas and Louisiana a foretaste of the hurricane with the eye of the storm still about 220 miles southeast of Galveston.

In its 5 p.m. EDT advisory, the National Hurricane Center said Rita's core was located 155 miles east-southeast of Galveston, Tex. It said the storm was packing sustained winds up to 125 mph and was moving northwest near 12 mph.

"On this track, the core of Hurricane Rita will make landfall along the southwest Louisiana and upper Texas coasts near daybreak Saturday," the hurricane center said.

It said hurricane winds extended outward up to 85 miles from the hurricane's center and tropical storm force winds extended outward up to 205 miles.

Some parts of southeast Louisiana were getting hit with sustained winds of 55 mph and gusts of 65 mph Friday morning.

Tropical storm warnings were up for a massive area from the southeastern coast of Louisiana, including metropolitan New Orleans and Lake Pontchartrain to Brownsville, Tex., encompassing populations of 2 million people as well as the heart of the nation's petroleum-producing and gasoline-refining center.

A portion of the affected area is just in the preliminary recovery stage from Hurricane Katrina, more than three weeks after it struck.

Rita changed direction overnight to northwest from west-northwest, and is forecast to hit just west of the Louisiana-Texas border.

The storm is expected to slow significantly once making landfall, threatening the region with rain accumulations as high as 25 inches over the next several days. Coastal storm surge flooding of 15 to 20 feet is expected as well.

Current National Hurricane Center models show the storm track ultimately angling to the northeast after landfall, with hazardous conditions extending well into northeast Texas near the Arkansas border.

Staff writer Fred Barbash contributed to this report.