Clark tweeted the news Sunday and referred to it as arson.?
Clark also retweeted a Jan. 30 tweet from the Cleveland Fire Department noting that the bodies of four victims from a fire on the city's east side had been located and recovered.
"The entire Seahawks family is mourning with Frank Clark and his loved ones after their tragic loss," the team said Sunday in a statement. "Our thoughts and prayers will be pouring out to Frank and his family during their time of grieving."
Cleveland.com reported that the fire began at 1 a.m. Tuesday, and investigators are trying to determine the cause of the fire. The report did not specify whether it was arson.?
Family members identified the victims to Cleveland.com as Alfonso Lathan Jr., 46; his 3-year-old son, Alfonso Lathan III; his 8-year-old granddaughter, Nyiah Lathan; and his 44-year-old nephew, who was not named. Alfonso Lathan's wife, Gianna Lathan, jumped out of a second-story window to escape the fire.
Seahawks teammate DeShawn Shead tweeted his condolences to Clark.
Clark's agent, Erik Burkhardt, offered his support as well.
Clark, 24, was a second-round pick by the Seahawks out of Michigan in 2015. He grew up in Los Angeles until age 12, when his mother sent him to live with family members in Cleveland.
Clark has 22 sacks in three seasons with Seattle. He became a starter in October 2017 after Cliff Avril suffered a season-ending neck injury.