(CNN)-- A murder and kidnap suspect may believe he's the father of the two young girls he's accused of abducting, his mother-in-law said Thursday.
"He believes they are his children," Josie Tate told HLN's Nancy Grace.
Adam Mayes, a Mississippi fugitive, is accused of killing the girls' mother and her 14-year-old daughter.
In a tearful plea to Mayes, Tate pleaded on HLN for him to return home Alexandria Bain, 12, and Kyliyah Bain, 8, and then turn himself in.
"You've had a chance to live life. They haven't," she said. "Give them that chance."
Authorities continued their search Thursday for the two Tennessee girls and Mayes.
The Mississippi man has been charged in the killing of Jo Ann Bain and her eldest daughter, Adrienne, 14, but authorities suspect he is still holding Bain's younger daughters.
He and his wife, Teresa, each have been charged with two counts of first-degree murder and two counts of especially aggravated kidnapping. He faces an additional count of making a false report, according to arrest affidavits filed in Tennessee.
In an interview with police, Teresa Mayes said her husband intended to take Alexandria and Kyliyah from their home in Hardeman County, near Memphis.
The pair first met as children when they were neighbors while living in Florida, Tate said. Then in 2001, Adam and Teresa reunited and eventually married.
Teresa Mayes' lawyer, Shana Johnson, said her client is cooperating with police but would not say whether she knows the whereabouts of Adam Mayes or the girls. The Mayes family and the Bain family are also connected through Adam Mayes' sister Pamela, who used to be married to Jo Ann's husband, Gary Bain, the lawyer said.
"We wish someone would call and help us," FBI spokesman Joel Siskovic said Thursday. "At this point, this guy doesn't want to be found, and we're out there trying to find him."
A day earlier, the FBI put Adam Mayes on its list of 10 most wanted fugitives and added $100,000 to the reward fund offered for a break in the case.
"There has been a lot of information that has come to light, and we want to make sure these girls are not brought to further risk. All of these details are coming in and going into the investigative picture," Siskovic said.
The search is concentrated around Guntown, Mississippi, where the bodies of the mother and daughter were found over the weekend.
"We are moving forward with our investigation to hunt down Adam Mayes and rescue those two little girls," Aaron Ford, the special agent in charge of the FBI's Memphis office, told reporters Wednesday. He urged Mayes to leave the girls in a safe location, such as a police station, church or hospital, "and then peacefully and safely turn yourself in."
The federal reward brings the total amount of money offered for information leading to Mayes' arrest to $175,000, Ford said. Mayes' wife, Teresa, and his mother, Mary Frances Mayes, are in custody, but Adam Mayes continues to evade a small army of 17 law enforcement agencies and hundreds of searchers.
The two young sisters were the targets of a kidnapping plot that involved the killing of their mother and older sister, a Tennessee court document says.
Police said Teresa Mayes told them she was in the Bains' garage where Adam Mayes killed Jo Ann and Adrienne Bain.
Details about the time or cause of death haven't been released. But in the affidavits, investigators said Adam and Teresa Mayes drove the bodies to Union County in northern Mississippi, where they were discovered Saturday in a shallow grave behind the house of Adam Mayes' mother in Guntown.
The mother, Mary Frances Mayes, has been charged with four counts of conspiracy to commit especially aggravated kidnapping.
Authorities describe Adam Mayes as a white man with blue eyes and brown hair. He weighs 175 pounds and is 6 feet, 3 inches tall. He recently cut his hair and may have done the same to the missing girls.
In security camera video shot April 30 at a convenience store near his home, Mayes is seen with short hair and a goatee. Nick Bargouthi, the clerk seen in the video, told HLN's "Issues With Jane Velez-Mitchell" that Mayes was a frequent customer.
"He walked in, went in and bought a Coke," Bargouthi said. "I noticed he cut his hair, got rid of his pony(tail). When I asked him, he said, 'Yeah, it was too hot.' But he kept all his facials on. And then he walked out."
He was last seen May 1 in Guntown. While the search is centered around his hometown, he also has connections to Arizona, Texas, Florida and the Carolinas, the FBI said.
Teresa Mayes' attorney Shana Johnson, said her client is cooperating with police but would not say whether she knows the whereabouts of Adam Mayes or the girls.
Adam and Teresa Mayes have been married for 11 years and lived in Guntown, she said. Johnson said she has asked for a mental health evaluation of her client.
Bobbi Booth, Teresa Mayes' sister, spoke to CNN's Anderson Cooper this week about Adam Mayes and his relationship with her sister, who she said has mental health problems.
"I've known Adam for at least 25 years, and he's always been weird and unusual ... but I never dreamed he would do something like this," she said.
She described him as aggressive and not trustworthy. He beat and threatened to kill her sister, said Booth, who pleaded with him to "do the right thing."
"Just let the children go. This has gone on way too far, and we need to figure out what's going on, and it's not fair to the children," she said.
Authorities tried to interview Mayes soon after Gary Bain reported his wife and the girls missing April 27 in Whiteville, a western Tennessee town of 4,600 people. However, Mayes fled, officials said.
Mayes may be using the alias of Christopher Zachery Wylde or Paco Rodrigass, his Facebook profile name, according to the bureau.
Ellen Fulghum, a guidance counselor at the high school Adrienne Bain attended, told HLN there was "a dark cloud" over the school and community.
"It's just a heaviness. Our hearts are heavy," she said.
"It's one of those situations where, as adults there's no way we can wrap our minds around something like this," Fulghum added. "It's very hard to help children through a grieving process when it is something of this nature."