GUNTOWN, Miss. — A Mississippi man killed a Tennessee mother and her teenage daughter so he could abduct two young sisters who are still missing, according court documents filed Wednesday, and a relative says the suspect thought the two younger girls might be his daughters.

The developments gave the first hint of a motive in the case that began in southwest Tennessee, stretched into Mississippi and led the FBI to put 35-year-old Adam Christopher Mayes on its 10 Most Wanted list. Authorities said they think the missing girls, 12-year-old Alexandria and 8-year-old Kyliyah Bain, are still with Mayes nearly two weeks after he took them.

Mayes' mother-in-law told The Associated Press he thought he might be the girls' father and it caused trouble in the marriage to her daughter, who's jailed in the case.

"She was tired of him doting on those two little girls that he claimed were his," Josie Tate said in an exclusive phone interview with The Associated Press on Wednesday. In an earlier interview, Tate's daughter Bobbi Booth said Teresa Mayes suspected her husband was having an affair with Jo Ann Bain.

Authorities refused to comment on the motive for the April 27 slayings and abductions at a Wednesday news conference.

Mayes and his wife, Teresa, were charged in Bolivar, Tenn., on Wednesday with first-degree murder in the deaths of Jo Ann Bain and 14-year-old Adrienne Bain. Their bodies were found buried outside the Mayes' home near Guntown, Miss., a week after they were reported missing by Jo Ann Bain's husband, Gary.

Teresa Mayes told investigators she saw her husband kill the two in the garage at the Bain home near Whiteville, Tenn., and then she drove him, the younger girls and the bodies to Mississippi, affidavits filed in court say.

The mother and older girl were killed so Mayes could take the younger sisters, the affidavit said.

Federal authorities pleaded for the public's help in finding the sisters and urged Mayes to surrender.

"Turn the girls in, and then peaceably and safely turn yourself in to law enforcement," FBI Special Agent Aaron Ford said at the news conference.

Officials said the reward for information "leading directly to the arrest" of Mayes is now $175,000.

Since the manhunt began for Mayes, people who knew him and the Bains have described him as unusually close to the family and the girls. He was described as a friend of Gary Bain, and the children considered him an uncle.

He was often at the Bain home. Authorities said he was spending the night there before the mother and daughters disappeared so he could help the family to pack for a planned move to Tucson, Ariz., and then drive their belongings west.

A Facebook page that Adam Mayes was using under an alias confirmed by law enforcement showed several photos of him and the Bain girls. One picture depicts Mayes and two of the girls smiling, all leaning next to one another cheek to cheek. The Facebook page has since been taken down.

Authorities said Alexandria has brown hair and hazel eyes and is 5 feet tall and 105 pounds. Kyliyah has blonde hair and brown eyes and is 4 feet tall and 57 pounds.

Sixteen-year-old Megan Ervin of Whiteville said at a Tuesday night vigil for the victims that she was a softball teammate of Adrienne Bain last year. She said Mayes spent time at the ballpark, often coming to see the Bain girls play.

"He was always here and everybody saw him," Megan said. "He was around all these kids all the time."

Ervin said she never spoke to Mayes.

"When I first saw him, I kind of had a bad vibe about him, so I just kind of stayed away," Ervin said. "But then I saw him here all the time and I figured he's no threat to us because he's always here.

"Obviously, that wasn't true."

Mayes' mother-in-law, who lives in Chatsworth, Ga., said she's known him for 25 years but didn't approve of him because his family never seemed to stay in one place and he couldn't hold down a job.

"Teresa's father and I begged her: 'Do not marry him, do not go off with him, do not live with him," she said.

Tate described her daughter as a slow learner who spent her school life in special education. Teresa Mayes was also incapable of having her own children, she said.

The mother said she believed Mayes had threatened her daughter and perhaps his own mother, Mary Frances Mayes, who has also been charged with conspiracy to commit especially aggravated kidnapping.

Mary Mayes' attorney, Somerville attorney Terry Dycus, said his client maintains she is not guilty. Dycus said it was too early to discuss what the mother's defense would be.

"She agrees with the authorities that he's possibly dangerous, but her main concern is that the children be returned immediately and safely," Dycus said.

Teresa Mayes faces six felony counts in the case: two first-degree murder charges and four especially aggravated kidnapping charges.

"The feelings I have for Adam are as close to hate as I'll ever come because he's destroyed not only the Bain family but he's destroyed my family too," Tate said.

Burke reported from Nashville, Tenn.