Citing sources with knowledge of the investigation, The Mercury News of San Jose on Monday reported that Foster's longtime girlfriend claimed that he dragged her during an argument at a Los Gatos home Sunday morning.
According to the San Francisco Chronicle, Foster was booked Sunday at the Santa Clara County Jail and is now being held without bail.
The Los Gatos Police Department did not respond to calls for comment.
"Upon arrival, officers spoke to Foster and the alleged victim". Foster, who was released Sunday after posting $75,000 bail, was booked on a single count of possession of an assault weapon. He was released on bail Sunday night.
But multiple sources familiar with the investigation told this news organization that police found a single SIG Sauer 516 short-barreled rifle, or SBR.
But at least the San Francisco 49ers appear to be taking things seriously.
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This is Foster's second arrest in the past month, as he was popped for marijuana possession in Alabama on January 12. He was released on $2,500 bond.
Cowboys owner Jerry Jones accused NFL commissioner Roger Goodell of overcompensating for a league scandal in 2014, when Goodell admittedly mishandled the suspension of former Baltimore Ravens running back Ray Rice back.
Last week, the San Francisco 49ers would make Jimmy Garoppolo the highest-paid player in the NFL.
But Foster now could face discipline from both the Niners and the National Football League, which could levy a fine or suspension under its personal conduct policy.
Aside from Foster's run-ins with the law, Lynch and Shanahan haven't had to deal with many off-field incidents since their arrival a year ago.
Foster has been arrested for the 2nd time in less than a month, according to ESPN's Nick Wagoner.