OKMULGEE, Okla. (KFOR) – Police in eastern Oklahoma said they have discovered human remains in their search for four missing men who were last seen riding their bikes last Sunday.

Investigators have been canvassing the area around Okmulgee, Oklahoma, all week looking for 32-year-old Mark Chastain, 30-year-old Billy Chastain, 32-year-old Mike Sparks, and 29-year-old Alex Stevens, who were reported missing on Monday by their families.

The Okmulgee Police Department said the four were close friends. They were believed to have left Billy Chastain’s house on the west side of Okmulgee on Sunday night, and were all reportedly on bicycles.

The remains were found Friday afternoon in a river southwest of Okmulgee when a passerby told police they noticed something “suspicious” protruding out of the water.

“Officers responded and discovered what appears to be multiple human remains in the river [but] we don’t know that this is our missing men yet,” said Okmulgee Police Chief Joe Prentice.

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Even though the remains have not been positively identified, Prentice said the missing men’s families were informed of the discovery out of respect.

“If it turns out that these four missing men are the remains in the river, then the focus of our investigation will shift from finding them to what happened to them [but] if it’s not them, then we’re back to square one,” Prentice continued.

Multiple agencies, including the Oklahoma Highway Patrol Boat Team, were at the river to assist with the recovery process.

Phone data previously suggested that the men may have ridden out of town in the opposite direction of the river, so police said they hadn’t considered the area where the river is located.

“We have more questions than answers,” said Prentice in a press conference Friday.