Detectives investigating the disappearance of 15-year-old Sierra LaMar now believe the Morgan Hill teenager was likely abducted, the Santa Clara County Sheriff's Office announced Tuesday afternoon.

After interviewing those who know her best, investigators believe it is highly unlikely that LaMar, who went missing a week and a half ago, ran away from home, sheriff's spokesman Sgt. Jose Cardoza said.

The sheriff's office was asking local residents to report any suspicious behavior they observe in co-workers, neighbors or other community members.

Such activity could include changing one's appearance, cleaning a vehicle in an extreme way, missing days of work, or showing an unusual interest in or avoidance of news reports on the case, Cardoza said.

On Tuesday morning, hundreds of volunteers from throughout the Bay Area showed up at Burnett Elementary School in Morgan Hill to participate in an organized search for Sierra.

The KlaasKids Foundation, which provides search-and-rescue services for families of missing people, was on hand to manage the search.

By 9 a.m., more than 200 people had registered to help look for LaMar or provide administrative support, KlaasKids founder Mark Klaas said.

Klaas' 12-year-old daughter, Polly Klaas, was kidnapped from her Petaluma home in 1993. Searchers looked for Polly for about two months before police arrested a suspect in the case, Richard Allen Davis, who led them to her body. Davis was later convicted of her murder. 

Volunteers stood in a long line that wound around Dougherty Avenue that morning as they waited to be checked in at the school.

"This is an amazing turnout," Klaas said, glancing around the school's multipurpose room, which was converted into a makeshift command center Tuesday.

"We're lucky if we get this many people during an entire search," said Klaas.

KlaasKids, which in 2011 alone provided search-and-rescue services in more than 200 missing persons cases nationwide and helped to close 37 of those cases, was summoned by Sierra's parents last week to assist in their case, Klaas said.

"They asked us if we would help them find their daughter," said Klaas.

Klaas said three search-and-rescue experts flew in from Florida and Texas over the weekend to help prep for the search efforts. KlaasKids is teaming up with similar organizations, including Bay Area Missing and the Laura Recovery Center, for the Sierra LaMar case.

One Bay Area Missing volunteer who turned out in Morgan Hill Tuesday morning was Michael Le, whose sister, nursing student Michelle Le, disappeared from Kaiser Permanente Medical Center in Hayward last May.

Her body was found in a remote area between Pleasanton and Sunol in September, and a former high school classmate and friend has been charged with her murder.

KlaasKids helped in the search for Le, which included hundreds of volunteers.

"I wanted to do something to pay it forward and be able to do the same for another missing person's family the way they helped us," Michael Le said Tuesday morning.

Volunteers, who were required to be at least 18 years old, were processed in the multipurpose room and some were given orange vests. They were organized into teams of about 15 people before fanning out to scour the area.

Many of Tuesday's volunteers waited outside in line for 30 minutes or more, clutching cups of coffee and chatting with each other.

Several people waiting in line Tuesday morning said they do not know LaMar or her family, but felt the urge to get involved after hearing of her disappearance.

"We just wanted to help ... it's way too close to home," said 22-year-old Vanna Ruiz, of Morgan Hill. "If that was my family members, I would want the whole world looking."

Several volunteers said that as parents, they wanted help Sierra's family cope in any way possible.

"It's a mother's worst nightmare," said Gilroy resident Jeannie McDevitt, 49.

McDevitt, who has three teenage sons and an 11-year-old daughter, said Sierra's disappearance has been a reminder to her and "a lesson for my kids" to be aware of their surroundings.

While many of the volunteers were from Gilroy or Morgan Hill, others woke up early to drive from as far as Pleasanton and Capitola.

LaMar left home for school the morning of March 16 but never showed up to class.

Investigators later found her phone and her Juicy-brand purse containing a neatly folded pair of pants and a T-shirt belonging to the teen.

The phone and purse were found separately about two blocks apart along Santa Teresa Boulevard, less than two miles from Sierra's mother's home.

Klaas said that after reviewing the evidence, he believes Sierra was likely abducted on the street where she lives.

"Whatever happened to Sierra happened on that block," he said.

Klaas said his and other missing persons groups hope to aid in the investigation by working with law enforcement and Sierra's family to uncover the information it will take to find her.

Volunteers were prepared to continue search-and-rescue efforts in Morgan Hill throughout the week, Klaas said Tuesday.

Anyone with information about the case was asked to call sheriff's investigators at (408) 808-4500 or the anonymous tip line at (408) 808-4431. Tips can also be emailed to tips@sheriff.sccgov.org