A set of missing persons reports has turned into a murder investigation after dismembered bodies found in an Oklahoma river were identified as four men who vanished last week, police said.

The men -- Mark Chastain, 32; Billy Chastain, 30; Mike Sparks, 32; and Alex Stevens, 29 -- were reported missing by relatives last Monday night or early Tuesday after leaving Billy Chastain's Okmulgee, Oklahoma, home on bicycles around 9 p.m. on Oct. 9, police have said.

The men appear to have been planning to commit a crime when they set out, Okmulgee Police Chief Joe Prentice said Monday, citing a preliminary investigation. All had been shot, he said.

Meantime, a person police want to talk with related to the case has been missing as of Saturday night and could be suicidal, the chief said. The person is not a suspect, and no charges have been filed, he said.

"I've worked over 80 murders in my career. I have worked murders involving multiple victims. I have worked dismemberments, but this case involves the highest number of victims, and it's a very violent event," Prentice said.

Here's what we know about the case:

ALL THE MEN FOUND IN THE RIVER HAD BEEN SHOT

A passerby called police Friday after seeing something suspicious in a river outside Okmulgee, a city of about 11,000 people roughly a 35-mile (56-kilometre) drive south of Tulsa, Prentice previously said.

Police arrived, saw remains and found multiple bodies.

The bodies were identified as the four missing men -- all having been shot and dismembered, the chief announced Monday.

"Although the official cause and manner of death is still pending, each victim suffered gunshot wounds," Prentice said. "All four bodies were dismembered before being placed in the river, and that is what caused difficulty in determining identities."

The remains were recovered over several days, and it's not clear what kind of evidence investigators will be able to recover.

"Whenever water is involved, it makes it much more difficult to identify evidence," the chief said.

Police have not recovered any bicycles, nor the gun used in the killings.

POLICE BELIEVE THE FOUR FRIENDS WERE PLANNING A CRIME

While it remains unclear exactly what led up to the men's deaths, it appears they were planning to commit a crime when they left Chastain's home, Prentice said, citing a preliminary investigation.

The belief is based on "information supplied by a witness who reports they were invited to go with the men to quote, unquote, 'Hit a lick big enough for all of them,'" the chief said Monday in a news conference.

"That is common terminology for engaging in some type of criminal behaviour, but we do not know what they were planning or where they planned to do it," Prentice added.

After the men were reported missing, police said they believed at least two of the men had cell phones with them. Investigators traced the phones' path, finding the devices went to two salvage yards -- one about five miles from the river, and the other about 10 to 12 miles from the river, Prentice told CNN on Saturday.

Police later found "evidence of a violent event" on property adjoining one of those locations, the chief said without elaborating.

A 'PERSON OF INTEREST' IS MISSING

Police have a person of interest in the case, and no charges have been filed, the chief said Monday.

The person, who has denied knowing the four men, is not a suspect but "a person of interest that we'd like to talk to," Prentice said.

But the person has gone missing as of Saturday night and could be suicidal, Prentice said.

Now that the case is a murder probe, police will be more tight-lipped about further proceedings "to protect the integrity of the investigation," Prentice said.