CALUMET, Oklahoma - Police have made an arrest in the brutal murder of a mother and her 5-year-old son in Calumet.

On June 16, the bodies of 22-year-old Amy Gibbins and her son, Bryor, were found in their burned down Calumet home. OSBI has now determined Amy was killed by blunt force trauma and Bryor died of smoke inhalation.

On June 25, Investigators arrested Derek Posey in the Tulsa area for the murders. Posey worked the oil fields around Calumet and was an acquaintance of Amy's according to the OSBI.

Investigators say Posey is spotted on surveillance video using her ATM card to withdraw money at an El Reno bank on June 16. The video is from around the same time the 911 call came in for the fire, according to OSBI.

Investigators say Posey also took Gibbins' cell phone which helped them track Posey to the El Reno ATM.

"It's a tough job, especially when you work something like this," says Calumet Police Chief Brian Huckabee who knew the victims.

"She was a good mother. She was a good friend to many. She was a good person," says Chief Huckabee about Amy Gibbins.