Brennan's father, Terry, told KITV-TV in Honolulu that his son suffered broken ribs, a broken left collarbone and possible head trauma in the crash. Colt Brennan was taken to Kona Community Hospital after the accident, then transported to Honolulu for further medical treatment, his agent, Ryan Tollner, said in a statement.
Terry Brennan later told the Honolulu Star-Advertiser that his son was in stable condition and had been sedated during the trip from Kona to Oahu, although The Queen's Medical Center still listed the former quarterback's condition as serious.
"This is our worst nightmare," said Terry Brennan, who planned to travel from his Irvine, Calif., home to Hawaii on Saturday morning. "You always worry about safety, whether you're in Hawaii or on the mainland.
"We're upset," he added. "We're hopeful all of the victims will be OK."
Brennan, 27, was a passenger in a sport-utility vehicle driven by his girlfriend, Shakti Stream, when the 27-year-old Kona woman crossed the center line on Queen Kaahumanu Highway near Makalawena around 9:30 a.m. local time and collided head-on with a sedan driven by a 47-year-old Waikoloa woman, police said.
Stream also was taken to Kona Community Hospital for treatment on a broken collarbone, the Star-Advertiser reported. The unidentified woman was transported to North Hawaii Community Hospital.
Police opened two negligent injury investigations. The cause of the crash was under investigation, and it wasn't known if speed, alcohol or drugs were factors in the crash, police said.
The Star-Advertiser cited a source in reporting that Stream told police her brakes malfunctioned. A source said speed wasn't a factor in the crash.
Brennan, who finished third in the voting for the 2007 Heisman Trophy while at the University of Hawaii, was the Redskins' 2008 sixth-round draft pick. He was released by the Redskins before the start of training camp this year. He then was signed and later released by the Oakland Raiders, and last month, he had a workout with the Houston Texans.
Brennan developed a solid fan base in Washington despite never playing in a regular-season game in two seasons. He spent 2009 on injured reserve after hip surgery, then had knee surgery in March, which set him further back.
While at Hawaii, Brennan became the NCAA career leader in touchdown passes, throwing 131 from 2005 to 2007.
Hawaii associate head coach Rich Miano told KGMB-TV and KHNL-TV that the team would be thinking of Brennan when it hosts San Jose State at Aloha Stadium on Saturday.
"For years, we've known Colt Brennan -- all we've seen is triumph and victories, record-setting and smiles, always saying the right things, just positive things for the state of Hawaii," Miano said. "So this is obviously kind of a sad event."
The Associated Press contributed to this report.