OKLAHOMA CITY (AP) -- Four people are being held as material witnesses in the investigation of the shooting

death of Oklahoma Highway Patrol Trooper Nikky

Green, authorities said Wednesday.

Former Duncan firefioghter Rick Ray Malone is the only suspect in Green's death and will likely be charged next week with murder, the Oklahoma State Bureau of Investigation says.

Being held as material witnesses are Tyson Wayne Anthony, Tammy Sturdevant, James Rosser and Jamie Marie Sowell were arrested Tuesday in the Lawton and Duncan areas, said OSBI spokeswoman Jessica Brown.

The ages and hometowns of the four were not immediately available. They were being held in the Lawton City Jail pending transfer to the Cotton County Jail.

Although the four were being held as material witnesses, that does not mean they witnessed the shooting, Brown said. She declined to elaborate.

Malone remains jailed in Stephens County where he was initially arrested on drug charges Sunday.

He complained of chest pains Tuesday and was taken to Duncan Regional Hospital.

Sheriff Jimmie Bruner said that while at the hospital, Malone was seen moving something in his mouth and swallowed the item when ordered to open his mouth, telling police it was a ring.

X-rays of Malone's chest and abdomen showed what appears to be a key to police handcuffs, Bruner said.

The sheriff said Malone will be kept in a one-man cell at the jail and under constant supervision.

Green was shot and killed last Friday while investigating a suspicious car on a rural Cotton County road.