Grapevine police spokesman Sgt. Robert Eberling said Wednesday night that the alleged assault took place early Sept. 20 at the Gaylord Texan Resort and Convention Center.
The team flew to St. Louis later that Saturday, and Spillman played against the Rams the next day.
He has played in all four games, including last weekend at home against New Orleans. His only tackle of the season came against the Rams. Spillman is listed as a safety but hasn't played defence this season.
Spillman's agent, Ron Slavin, and attorney Bruce Ashworth declined to comment. The Cowboys didn't immediately respond to an email seeking comment.
Eberling said no other details were being released because the investigation is ongoing.
The Cowboys signed the 28-year-old Spillman on Sept. 1, two days after the sixth-year player was among the final cuts by San Francisco. The former Louisville defensive back spent most of his first five seasons with the 49ers.
The alleged assault came less than 24 hours after NFL Commissioner Roger Goodell held a news conference to address concerns about the way the league has handled domestic violence cases.
"Obviously, we see what's going on with the NFL, but these things are very sensitive in nature when it comes to the victim," Eberling said. "We don't release any details involving these type of crimes while they're under investigation."
Goodell and the league were widely criticized over a two-game suspension for Baltimore running back Ray Rice after he knocked out his then-fiancee in a hotel elevator.
The commissioner later acknowledged the punishment was too light and said future first offences in domestic violence cases would result in six-game bans. When an explosive video surfaced showing Rice punching the woman, he was released by the Ravens and suspended indefinitely by the league.
The NFL also has been rocked by a pair of assault cases last month that sidelined running backs Adrian Peterson in Minnesota and Jonathan Dwyer in Arizona.
Peterson was indicted in Texas on felony child-abuse charges for using a tree branch, or "switch," to discipline his 4-year-old son.
Dwyer won't play again this season after his arrest on aggravated assault charges. A law enforcement report says he head-butted his wife and broke her nose after she refused his sexual advances, and punched her in the face the next day.
Peterson and Carolina defensive end Greg Hardy are on a paid leave through a special roster exemption granted by the league. Hardy is appealing his conviction in a domestic violence case involving an ex-girlfriend in North Carolina.
San Francisco defensive lineman Ray McDonald is playing after he was arrested on suspicion of domestic violence at a home he shares with his fiancee. He hasn't been charged.
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