An escaped inmate and a guard suspected of helping him had a "special relationship", say investigators.
The announcement from the Alabama sheriff's office said the discovery was confirmed by fellow inmates of missing prisoner Casey White, a murder suspect.
A manhunt is underway for the 38-year-old escapee and corrections officer Vicky White, 56 (no relation).
The pair vanished as she was transporting him to a bogus mental evaluation.
Police have not revealed whether the relationship was romantic.
'Nothing to lose'
Friday - the day the pair went missing - was supposed to be her final day at work. She had recently sold her home and told colleagues she planned to spend more time at the beach.
Police now believe Ms White helped orchestrate the escape after breaking jail protocol to escort Casey White alone.
"Investigators received information from inmates at the Lauderdale County Detention Center over the weekend that there was a special relationship between Director White and inmate Casey White," Sheriff Rick Singleton said in a statement on Tuesday.
"That relationship has now been confirmed through our investigation by independent sources and means."
On Tuesday, the US Marshals Service said the two were last seen on Friday driving a copper-coloured Ford Edge with an Alabama registration plate.
The same bulletin gave two aliases for Ms White - April Davis and Renee Marie Maxwell. Authorities say Casey White should be considered "armed and extremely dangerous".
Officials say they may now have access to an AR-15 rifle and a shotgun.
On Monday, Sheriff Singleton described the 6ft 9in Casey White as "extremely dangerous" and advised law enforcement not to "take any chances" with him.
"He has nothing to lose," he said.
From 'solid employee' to 'wanted fugitive'
He added that Ms White is thought to be using cash, and has switched off her mobile phone.
The US Secret Service is helping to track her financial records, he said.
"I would have trusted Vicky with my life. I really mean it," Lauderdale County District Attorney Chris Connolly told reporters. "If we needed something from the jail, she was our go-to person, solid employee. That's why it's so shocking."
The Marshals Service called Ms White a "wanted fugitive".
Casey White was charged in September 2020 with murder over the stabbing of 58-year-old Connie Ridgeway. He was already serving 75 years for a series of violent crimes in 2015, including burglary, vehicle theft and a police chase.
He allegedly confessed to the murder, but later pleaded not guilty by reason of insanity, and was awaiting trial at the Lauderdale County Jail when he disappeared, authorities added.
He faces execution if convicted.
Meanwhile, a woman who White tried to kill during a burglary said she is "freaking out" after discovering he is on the loose.
The unnamed woman told local ABC affiliate WAAY 31 that she and her family are in hiding, fearful that White will track them down.
She also said she has a message for Ms White: "If she is still alive, get the hell out. Run. Run as far as you can, and turn yourself in, and contact somebody.
"Like, do the right thing before you lose your life or somebody else does."