Investigators searching for 4 close friends who vanished in Oklahoma

Police are searching for clues in the mysterious disappearance of four close friends in Oklahoma.

Mark Chastain, 32; Billy Chastain, 30; Mike Sparks, 32, and Alex Stevens, 29, were last seen leaving one of their homes in Okmulgee around 8 p.m. Sunday reportedly riding bicycles, according to the Okmulgee Police Department.

Both of the Chastain men and Sparks were reported missing by one of their spouses during the overnight hours on Monday, police said. Stevens' mother contacted authorities a few hours later also to report her son's disappearance.

The Okmulgee Police Department gathered video and additional GPS information and followed up on potential sightings on Thursday, but said they were unable to confirm any reported sightings of the men.

Police said they would spend the evening Thursday and all of Friday reviewing hours of video and GPS data. Police also said they were able to identify additional locations as potential sources for more video and will be contacting them Friday.

Police submitted search warrants for phone records and will be submitting warrants for Facebook accounts.

Police spent Tuesday and Wednesday interviewing potential witnesses, working to gather descriptions and canvassing areas to confirm reported sightings.

"These men are exhausted," Police Chief Joe Prentice said in a Facebook post shared on Wednesday. "I am sending them home to get rested and we will start fresh tomorrow, unless something time sensitive comes in overnight."

According to investigators, two of the men are believed to have cellphones with them, however, attempts to contact them go straight to voicemail.

Chastain's cellphone was tracked to an area south of Okmulgee, but was turned off or lost power at some point. Officers checked the area, but found no sign of the men, police said.

Anyone with information regarding their whereabouts is urged to contact the Okmulgee Police Department.