Police searching for person of interest in death of New Hampshire couple found in shallow beach grave

Police are searching for a person of interest in the death of a New Hampshire couple who was found buried on a Texas beach.

James Butler, 48, and his wife, 46-year-old Michelle Butler, were found buried in a shallow grave on a remote part of Padre Island along the southern coast of Texas on Oct. 27 and 28, nearly three weeks after they disappeared, police said.

The unidentified person of interest was seen driving the couple's stolen RV across a checkpoint into Mexico in surveillance footage on Oct. 21. There were no suspects in custody as of Sunday and police are still searching for the RV.

Authorities discovered the couple's badly decomposed remains after one of their cellphones pinged in the area. Deputies with the Kleberg County Sheriff's Office were following up on the lead on when they noticed a bra covered in sand on the side of a dune. Upon further investigation, they discovered the bodies.

The sheriff's office said it took several days to identify the bodies because they had no identification and had badly decomposed. Their causes of death have not been released, but police are investigating the case as a homicide.

The couple's family said the pair were avid travelers and were partaking in a cross-country road trip to Florida, where they planned to sell Christmas trees.

They were reported missing on Oct. 23 when family and friends notified police, saying they hadn't heard from them in a week.

"They are always in contact with the families. So, the fact that, even for a day, they were out of contact was really strange," James Butler’s niece, Stephanie van Loon, told ABC News in an interview Sunday. "They were the type of people to open their hearts and homes to everybody. They were just those kinds of people.

"They [met] great friends all along the way, which is part of the reason the social media factor was so helpful because everyone really wanted to help find them. They had friends all over the U.S. from their travels," she added.

Van Loon said Michelle Butler had children from a previous relationship and grandchildren she "always wanted to talk to." She described the Butlers as a fun-loving couple who enjoyed making people laugh and partaking in outdoor activities like horseback riding and hiking.

"I have so many photos of them just going all over the U.S. and all the hiking trails through all the different climbs. They were really loving the chance to explore out West and the desert environments," van Loon said. "Growing up here in New Hampshire, it’s obviously very deciduous and it’s just they were really trying to experience the parts of the U.S. that he hadn’t been able to experience before."

She said the family is looking forward to finally starting the grieving process, but she said they won't be able to get proper closure until justice is served.

"In a way, it was a sigh of relief for everybody because if we had found out that it wasn’t them, it honestly would have been so much worse in the sense of not knowing where they were or anything," van Loon said. "We’re hoping to find whoever is responsible and get an answer as to why something like this would have happened."

ABC News' William Mansell and Frank Elaridi contributed to this report.