"It happened so quick it didn't even look real," said Murray Lea, who witnessed the accident. "I'm walking back to my van and all of a sudden you hear, 'Vroom,' right off the edge of the cliff. It blew my mind."
Police said Seau was going approximately 60 miles per hour when he went off the cliff.
According to authorities, Seau said he had gotten sleepy.
Seau was alone in his Cadillac SUV. He was taken to Scripps La Jolla Hospital, according to Carlsbad police Lt. Paul Mendes.
According to the San Diego County Sheriff's Department, Seau was arrested on suspicion of domestic violence and booked into the Vista jail. He was booked around 2 a.m. and released about 3:20 a.m.
The football star's 25-year-old, live-in girlfriend told authorities that Seau assaulted her during an argument at his Oceanside home, according to police. She appeared to have minor injuries and did not require medical treatment.
The Associated Press and City News Service contributed to this report.