"Deputies were unable to get everyone off the bus," spokesman Don Peritz said. "We believe it's going to be closer to 20 fatalities."
The bus, carrying about 45 people, was engulfed with flames early on Friday, causing a 17-mile (27-km) backup on a Interstate 45, already heavily congested with evacuees from the Gulf Coast.
The bus was reduced to a blackened, burnt-out shell, surrounded by numerous police cars and ambulances.
There were indications that oxygen used by elderly evacuees could have had a role in the fire, Peritz said.
There were a series of explosions, apparently from the oxygen equipment, he said.
"The early indications are this is a mechanical issue. 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."
Interstate 45 stretches more than 250 miles (400 km) from Galveston through Houston to Dallas. The site is roughly 17 miles (27 km) southeast of downtown Dallas.