The Seahawks are aware that Thomas won't attend and have said there is "zero chance" of a new deal for the star safety if he does not report, according to Mortensen.
NFL Network first reported Thomas' plan to skip the start of camp.
Thomas took his dispute to Instagram last week when he posted that the Seahawks should "make a trade happen" if they don't intend to sign him to an extension. He also told ESPN at the Pro Bowl in January that he didn't see himself playing without a new contract.
He is entering the final year of a four-year, $40 million deal that pays him an annual salary that ranks sixth among safeties.
By holding out from training camp, Thomas is subject to up to $40,000 in fines for every day he misses. After five days, the team can also fine Thomas 15 percent of his signing bonus proration plus an additional 1 percent for every day he misses beyond that, up to 25 percent. Thomas' 2014 extension included a $9.5 million signing bonus, of which the proration for 2018 is $1.9 million. However, teams can choose to not impose those fines.
Thomas, 29, skipped Seattle's mandatory minicamp last month in protest of his contract situation. The three-time All-Pro was the NFL's highest-paid safety in terms of annual average when he signed his deal in 2014. The Kansas City Chiefs' Eric Berry now tops the list at $13 million per year.
With Thomas absent from offseason workouts, the Seahawks most often had Bradley McDougald in his place at free safety and Delano Hill at strong safety. They at times had McDougald at strong safety and Tedric Thompson at free safety.
The Seahawks have had discussions with the Dallas Cowboys about a possible trade that would send Thomas to his hometown team. Thomas, in a moment caught on camera following a Christmas Eve victory over the Cowboys at AT&T Stadium, memorably chased down Jason Garrett and told the coach to "come get me" if he were to become available.
Thomas and linebacker Bobby Wagner are two of the star defenders who remain with the Seahawks after an offseason of big-name departures. Seattle cut Richard Sherman, traded Michael Bennett, lost Sheldon Richardson in free agency and released Cliff Avril with a failed physical designation.
ESPN's Brady Henderson contributed to this report.