CONCORD, N.H. (AP) _ New evidence found in the search for two children missing since July Fourth has led authorities to believe they may be murder victims, the state's assistant district attorney said Saturday.

Police haven't found the bodies of Sara Gehring, 14, or Philip Gehring, 11, but the investigation has changed from a missing persons case to an apparent double-homicide investigation, Assistant Attorney General Jeffery Strelzin said.

Strelzin wouldn't say what specifically led authorities to believe the children are dead.

When their father, Manuel Gehring, 44, of Concord, was arrested in California on Thursday, the children weren't with him. Authorities have said he drove more than 3,200 miles from Concord to Gilroy, Calif., between July 4 and July 10.

According to court documents, the children were in tears when they and their father left an Independence Day fireworks display. The documents also say Gehring did not agree with a new custody arrangement between him and his ex-wife that had been reached a week earlier.

Strelzin said Gehring is being held on interference with custody charges and additional charges may be brought later.

``We've developed information through investigation in California, throughout the country, through the various agencies involved,'' Strelzin said.

He wouldn't discuss what evidence had been discovered, but he said officials continue to ask for help from people who might have seen the family or their green minivan.

Gilroy police and FBI agents described Gehring as cooperative, but said Saturday that he had revealed nothing about what had happened to the children.

``We're hoping to focus on that continued cooperation that will lead us to the location of these children,'' FBI Agent Arthur Balizan said.

Neighbors who know Gehring in New Hampshire described him as a quiet and responsible man.

``He was soft-spoken and polite, and Sara seemed to have a good relationship with her Dad,'' said Carolyn Edy, who has known the family for more than 10 years and used Sara as a babysitter. She last spoke with Gehring two weeks ago when he asked if she'd act as a reference for Sara, who was applying for a job.

Though Sara was occasionally upset about the divorce, Edy said she saw nothing that suggested any possibility of violence. ``I never saw anything other than a nice family,'' she said.