According to police, the photo depicts two brothers who attend Edmonds-Woodway High School apparently having sex with a third Edmonds-Woodway student - a 15-year-old girl.
A fourth student, who attends Kings High School in Shoreline, recently tweeted the photo. Now police are investigating all four teens.
Sgt. Mark Marsh with Edmonds police tells KOMO Newsradio the photo may violate state child pornography laws, due to the age of the participants.
"We have to establish what the age was, at the time the photo was taken, of all the participants and we'll go from that point," he said.
Both of the boys in the photo are reported to be older than the girl, and there's also a question of whether she was old enough to consent to sex.
Police began investigating the case after it was referred to them by the Edmonds Woodway School District.
Marsh said the school has taken disciplinary action against the two brothers in the photo. No arrests have yet been made.
A spokeswoman for the Edmonds Woodway School District said officials want to reassure parents that they are continuing to look into the allegations.
The fourth teen at Kings High School also is under investigation, Marsh said. Any information about him will be forwarded to the Shoreline Police Department, he added.