Hundreds of volunteers joined the search Saturday for a missing 7-year-old girl even as an arrest and the discovery of bloodstained evidence lent an increasingly grim sense to the task.

The volunteers, carrying bright red tags to mark potential evidence, set out early in the morning for the foothills and deserts east of San Diego for a sign of Danielle van Dam, whose gap-toothed smile adorns thousands of missing person fliers around the nation.

Danielle's parents, Damon and Brenda van Dam, said in an interview that they are encouraged by the arrest Friday of David Westerfield, a 49-year-old neighbor, on suspicion of kidnapping their daughter, but they urged volunteers to continue helping with the search.

"We've got to focus. We've got a job to do and that's to find Danielle," Damon van Dam said at the real estate office that serves as search headquarters.

A sign at the headquarters marked Saturday as the 22nd day of the search effort and urged volunteers to "be positive."

"There's still a little bit of hope that we will get her back," Damon van Dam said.

Danielle's parents discovered her missing the morning of Feb. 2. She was last seen the night before when her father put her to bed. Police believe she was abducted from her second-floor bedroom of the family's north San Diego home.

Westerfield, a divorced father of two grown children who lives two doors down from the van Dams, has a 1996 drunk driving conviction but no violent criminal history, police said.

Investigators began focusing on Westerfield, a self-employed engineer, shortly after the girl's disappearance. He was at the same bar where Danielle's mother, Brenda, went with friends while Damon van Dam stayed home with their daughter and two sons.

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

Authorities said they found traces of Danielle's blood in the motor home and on an article of Westerfield's clothing. They also confiscated child pornography from his home, according to San Diego Police Chief David Bejarano.

Westerfield, whose lawyer said he plans to mount a "vigorous defense," is in custody at the county jail and is expected to be arraigned Tuesday.

"I'm confident that they got the right guy," said Damon van Dam, 37, an engineer for the San Diego wireless technology firm Qualcomm.

But the arrest is only a step toward resolution of the case, the parents said.

"We haven't crossed the finish line. We have to find Danielle," Brenda van Dam said.

With that in mind, volunteers donned bright orange and green safety vests to continue the search. Before setting out they listened to a safety briefing that included warnings to watch out for rattlesnakes and to drink plenty of water to avoid dehydrating in desert temperatures that were expected to reach 80 degrees.

Many volunteers said they have children and felt compelled to join the effort.

"It just hit home," said Steve Cassarino, a father of two who got a bad case of poison oak while searching a rugged area last week. "It could happen to anybody, any time."

Another volunteer, Gary Lowe, said he decided to give up his Saturday because he can't bear the thought of the van Dams not knowing what happened to their daughter.

"My main motivation is the fear that she will never be found and they will have to live with that uncertainty for the rest of their lives," Lowe said.