PASADENA, Texas -- Texas EquuSearch volunteers searched Wednesday for a missing 3-year-old girl and 7-year-old boy who police said may have been killed by their father, KPRC Local 2 reported.

Pasadena police said Randy Sylvester Sr., who is being investigated for capital murder, is the sole suspect in the disappearance of his daughter, Denim, and son, Randy Jr.

The children were last seen between the swimming pool and tennis courts at a Pasadena apartment complex at 4025 Burke Road at about 3:45 p.m. Sunday. Their mother reported them missing at about 9:15 p.m.

Denim and Randy Jr.'s paternal grandmother, Alonda Sylvester, called Texas EquuSearch Tuesday for help.

"Grandma called me yesterday morning just sobbing, crying," said Tim Miller, founder of Texas EquuSearch. "We're willing to do anything that we can do. Law enforcement's doing their investigation and if it gets to that point, they're going to call us to come in and help and we'll bring all the resources we need."

About 100 searchers are looking in a five-mile radius around the apartment complex. They are using horses, four-wheelers and focusing on wooded areas, drainage ditches and bodies of water.

Volunteers sifted through garbage at a Waste Management facility to see if anything that could help find the kids was tossed in the trash.

"They're going to be going through every bag that comes out of the trucks," Miller said. "It's not a job that anybody likes. We just got to stay focused, keep our emotions out of it, work with law enforcement."

Nothing was found.

An unnamed aircraft equipped with a camera will fly over dense property at the request of police.

Because of high gas prices, a local company donated gas for volunteers' vehicles.

One volunteer drove 30 miles to help.

"We really want to bring them home to their families. They are just precious," she said.

Volunteer Rebecca Craigin said she came to look for the children because she has young children of her own.

"I have three children of my own," she said. "I talked to them yesterday and said, 'That's why it's so important parents watch their kids. Kids are a blessing ... no parents should have to wonder where are my kids.'"

Volunteers are still needed, as well as donations of food and water, officials said.

Police said Sylvester, 27, became a suspect after he allegedly told the children's mother that the children were dead and that she needed to move on.

Detectives said bloodhounds trailed Denim's scent from the tennis court to Sylvester's apartment and then to his car.

Sylvester was arrested Tuesday on unrelated charges, shortly after he became confrontational with the media.

"I'm feeling like I want to fight all y'all," said Sylvester to a group of reporters and photojournalists.

Sylvester was charged with resisting arrest and was held on outstanding traffic warrants from Bellaire. He is under investigation of domestic violence against his wife from an incident on Monday night.

The family moved to Houston from New Orleans after relocated from Hurricane Katrina.

Denim is 2 feet 5 inches tall, 35 pounds and was last seen wearing a pink V-neck T-shirt with brown trim, pink shorts and pink flip-flops. Her hair was braided with a pink ponytail holder.

Randy is 4 feet 1 inch tall, 55 pounds and was last seen wearing a blue and orange Florida Gators shirt, blue denim shorts and blue and white tennis shoes.

Police said an Amber Alert was not issued for the children because they did not meet the criteria for one. Investigators said they must believe that an abduction occurred, but they do not feel that was the case.

Anyone with information is asked to call the Pasadena Police Department at 713-477-1221.

The FBI, the National Center for Missing and Exploited Children, Montgomery County Search and Rescue and Houston Police Department are also participating in the investigation.