The 12-time Pro Bowler's white Cadillac SUV was found on the beach about 100 feet below the roadside, said Carlsbad police Lt. Kelly Cain. Cain said it wasn't a sheer cliff but was a significant, rough slope. Seau was taken to Scripps La Jolla Hospital in San Diego.
Carlsbad police dispatcher Matthew Pinell said late Monday alcohol and drugs are not suspected to be factors in the crash "at this time."
Seau's 25-year-old live-in girlfriend told authorities that Seau assaulted her during an argument Sunday night at his Oceanside home, according to Oceanside police. She appeared to have minor injuries and did not require medical treatment.
Seau, 41, was arrested outside his home about two hours after police arrived. He wasn't home when authorities got there, but they asked him to return when he called his girlfriend.
Seau posted bail and was released from the San Diego County jail in Vista.
"At this point we think he's fine, but we really don't have any comment," said Richard Doan, who answered the cell phone of Bette Hoffman, who works for Seau's foundation. "There are no injuries, but he seems to be a little shaken up."
Hospital spokeswoman Lisa Ohmstede declined to comment on Seau's condition, and it was unclear whether he remained hospitalized Monday night.
Seau was a star linebacker with the University of Southern California before going to the San Diego Chargers with the fifth pick overall in the 1990 draft. He led the Chargers to the Super Bowl following the 1994 season and remained in San Diego until being traded to the Miami Dolphins in 2003.
He left football briefly in August 2006 after 13 seasons with San Diego and three with Miami. He signed with the New England Patriots four days later and started 14 of the 27 games he played for them over two seasons. He retired after the 2007 season, then came out of retirement for the final four games in 2008. Seau played seven regular-season games in 2009, all as a backup, and made 15 tackles.