The Bellevue, Washington, Police Department said in a release that the 28-year-old Lynch was accused by an unspecified female victim regarding an alleged incident at a downtown Bellevue apartment unit at 2:30 a.m. PT.
According to the police report, no domestic violence or sexual assault was alleged and no arrests have been made.
"This is only an allegation," Bellevue PD spokesman Seth Tyler told the Seattle Times. "We haven't even verified that [Lynch] was involved."
Lynch began practice with the team on Sunday morning at 1:15 p.m. ET. The alleged incident occurred about eight hours earlier. The Seahawks players are off Monday.
Lynch, 28, missed the first eight days of training camp after holding out for changes in his contract, which is starting the third year of a $30 million deal.
Lynch returned after the Seahawks added $500,000 from 2015 salary to his 2014 money, along with guaranteeing $1 million in bonus incentives for 2014. That's raised his 2014 base salary from $5 million to $6.5 million, but team sources said Lynch wanted an additional $5 million in 2014.