SAN DIEGO (AP) — Investigators have concluded that a missing 7-year-old girl is dead, and prosecutors plan to file a murder charge against a neighbor that would allow them to seek the death penalty.

David Westerfield, 50, was to be arraigned Tuesday in the death of Danielle van Dam, a second-grader who vanished from her bedroom more than three weeks ago.

"I must conclude that Danielle van Dam is no longer living and was killed by her abductor," San Diego County District Attorney Paul Pfingst said Monday.

Westerfield's attorney, Steven Feldman, promised a "vigorous defense."

Westerfield, who lives two doors from the van Dam home, was arrested Friday and jailed without bail. Pfingst said there are no other suspects.

Authorities have said they found traces of Danielle's blood in Westerfield's motor home and on an article of his clothing.

Danielle was last seen Feb. 1 when her father put her to bed. Police believe she was abducted from her second-floor bedroom in the family's north San Diego home. An extensive search that has stretched from Mexico to the desert east of San Diego has failed to turn up any trace of her.

The family's Web site said the Van Dams are offering a $25,000 reward for information leading to the return of Danielle.

The prosecutor said he would file one count of murder with a so-called special circumstance — murder during kidnapping — that will carry the possibility of the death penalty or life in prison if Westerfield is convicted. Pfingst said no decision has been made on whether to seek the death penalty.

Pfingst said he had an emotional weekend meeting with Danielle's parents, Brenda and Damon van Dam.

"It was difficult to bring out the word 'murder,"' he said. "Both parents were in tears."

The van Dams declined to speak with reporters after Monday's announcement.

"They're maintaining their privacy but also want to thank anyone involved in the search. They're just continuing to try to find Danielle," family spokeswoman Sara Fraunces said.

Volunteer searchers were given instructions Monday on how to look for a body.

"That was pretty emotional for everybody," said George Rillo, a computer programmer from San Marcos who took the day off to help.

The absence of a body creates a legal challenge, but the district attorney's office has successfully prosecuted four such cases, the most recent in August.

To try someone for murder without a body, prosecutors must establish a "reasonable probability" that the victim has died, said Justin Brooks, director of the California Innocence Project at California Western School of Law in San Diego.

"It's nowhere near as hard as proving something beyond a reasonable doubt, but it's a lot more than just showing blood stains," Brooks said.

Westerfield, a divorced father of two grown children, has a 1996 conviction for drunken driving but no violent criminal history, police said.

Investigators began focusing on the self-employed engineer shortly after the girl's disappearance. He was at the same bar where Brenda van Dam partied with friends the night Danielle disappeared while her husband stayed home with their daughter and two sons.

By the next morning, Westerfield had set off for the beach and desert in his motor home and was the only one of the van Dams' immediate neighbors who was gone when the search began.