(CBS News) GUNTOWN, Miss. -- There's a new name on the FBI's Ten Most Wanted list.
Adam Mayes is accused of murdering a mother and daughter two weeks ago and taking the woman's two younger daughters.
An intense, multi-state manhunt continued Thursday.
The murder victims were found last weekend in Guntown, Miss.
Officials in Mississippi say they have reason to believe the two younger children of Jo Ann Bain are still with Mayes and are still alive.
Now, it's a race against time for authorities to find the man they're calling armed and extremely dangerous.
"We believe," says Aaron T. Ford, special agent in charge of the FBI's Memphis office, "Adam Mayes could be anywhere in the United States, and we are extremely concerned for the safety of the girls."
The FBI intensified its search for Mayes Wednesday by making him one of the most wanted men in America, putting him on its Most Wanted List.
He's suspected of killing Bain, a family friend, and her 14-year-old daughter, Adrienne, in the garage of their Tennessee home last month, then kidnapping 12-year-old Alexandria and 8-year-old Kyliyah.
Mayes' mother-in-law says he believed he was their father and was obsessed with them.
"My daughter said he lived, ate, sleep, and breathed nothing but these two children," Josie Tate told WTVC-TV.
Meanwhile, Mayes' own mother has been charged with conspiracy in the case.
Charges against Mayes and his wife, Teresa, were upgraded to murder Wednesday.
Teresa is said to claim she saw her husband kill in cold blood. According to court documents, Teresa told police both murders were directly part of the kidnapping of Alexandria and Kyliyah and that she drove Mayes, the two bodies and the two youngest Bain girls to Union County, Miss.
That's where Mayes was seen last - caught on surveillance video at a convenience store.
In the video, a store clerk asks Mayes if the weather is hot enough for him, and Mayes replies, "Yeah, but a little cooler, since I got a haircut."
Investigators again searched the area around Mayes' Mississippi home Wednesday, but say he could be hiding anywhere.
"I must emphasize that Mayes also has ties to Texas, North Carolina, South Carolina, Florida and Arizona," said Daniel McMullen, the FBI special agent in charge in Mississippi.
The reward for any information leading to Mayes or the girls has grown to $175,000.
"To Adam Mayes," says the FBI's Aaron T. Ford, "I would urge you if you're listening to, number one, turn those girls in to a safe location ... then peacefully and safely turn yourself in to law enforcement."
The search for Mayes is very slow-going - there's a lot of thick brush in the Guntown area.
But the FBI says authorities are getting lots of new tips and leads every day.
To see Mark Strassmann's report, click on the video in the player above.