‘A matter of time’ before dangerous fugitive caught in case of slain couple, expert says

iStock(NEW YORK) -- As authorities hunt for a man who they say stole a slain couple's RV and fled to Mexico, one expert says his capture is just "a matter of time."

Adam Curtis Williams, 33, who is considered armed and dangerous, is wanted in connection with the deaths of James and Michelle Butler, whose bodies were discovered in a shallow grave on Padre Island in Texas last week.

A felony theft arrest warrant was issued for Williams on Tuesday, authorities said.

Police say Williams is the man seen driving the Butlers' RV across a checkpoint in Mexico in surveillance footage from Oct. 21. Police believe he's with Amanda Noverr, who they say is the passenger seen in the footage.

The surveillance footage had been released before Williams and Noverr were identified.

"Thankfully they have two individuals who have been identified. They were chasing a ghost a few days ago, but I know the U.S. Marshals have teamed up with Mexican authorities and they are utilizing all their resources at this point," Lenny DePaul, a former chief inspector with the U.S. Marshals, told ABC News.

Investigators are now combing through the suspects' backgrounds -- including their social media accounts -- and are also likely examining home surveillance cameras and CCTV footage to help trace their route, said DePaul.

"Their trusted circle of friends is going to be very important with this case," DePaul added. "They’re on the run, they’re going to communicate."

"Timing is everything when you're chasing a human being, especially a fugitive," DePaul said. "It will be a matter of time before they can put this case to bed."

The remains of the Butlers were found on Oct. 28, one day after deputies, following up on a lead, noticed a bra covered in sand sticking out of the side of a dune, Kleberg County Sheriff's Lt. David Mendoza said Monday.

The New Hampshire couple was on a cross-country RV road trip and were last seen on Oct. 15 as they left the Padre Balli Park, Mendoza said. The next day, Michelle Butler posted to social media that they had "just arrived at their new home for the next couple of weeks" before they were to set out to Florida, Mendoza said.

"I think with the way the timeline played out, these two victims were missing for a couple of weeks. It’s not a straight-up carjacking," said DePaul. "And the million dollar question at this point -- what the motive was. No one seems to know that."

Williams has not been charged for the Butlers' deaths and no arrest warrant has been issued for Noverr, authorities said Tuesday. Both are believed to be from the Utah area, authorities said.

Anyone with information about their whereabouts is asked to call the Kleberg County Sheriff's Office at 361-595-8500.

Copyright © 2019, ABC Radio. All rights reserved.