DENVER, Colo. -- Denver police are searching for a 3-year-old girl they believed to be dead, stuffed in a large plastic bag and dumped somewhere in the Capitol Hill neighborhood.

On Sunday, about 200 volunteers and police officers canvassed downtown Denver between Sixth Avenue and 20th Street and Downing Street and Speer Boulevard.

Using search dogs, volunteers searched since 4 a.m. Sunday looking into Dumpsters and around the Cherry Creek and South Platte River for any sign of Niveah Gallegos.

"We went Dumpster by Dumpster, alley by alley, street by street," said police spokesman Sonny Jackson. "We were very systematic in the process."

"It's pretty disheartening to look for such a long time and not come up with anything," Jackson said.

Police continue to ask for help in locating Gallegos.

The public has been asked to look through Dumpsters for a plastic bag that might contain her body.

The Guardian Angels offered their help Sunday night.

"I've got to tell you I've heard a lot of support as we go people say, 'I've checked my Dumpster, keep moving on,'" said Nick Trammell of the Guardian Angels.

Businesses which were closed over the weekend have been asked to check their dumpsters on Monday morning.

Jackson said trash pickup for large Dumpsters will be suspended so police can continue to look for Gallegos. Smaller trash cans will be collected as normal, however.

On Saturday, top law enforcement officials issued a disturbing plea for help from the public.

"It's pretty simple," said District Attorney Mitch Morrisey. "Walk out into your back yard, look through your Dumpsters, help us find this little girl. She's probably in a black plastic bag or white plastic bag."

The girl's mother, who had claimed on Friday that her little girl was snatched from her arms by two men while she was walking down an alley, is now being held on suspicion of being an accessory to the slaying.

"We've arrested the mother as an accomplice. We've also arrested the mother's boyfriend who we believe may have done something with this child," said Denver police spokesman Sonny Jackson.

Miriam Gallegos, 20, was arrested Friday night.

Her boyfriend, Angel Ray Montoya, was taken into custody Friday evening after an all-night manhunt. He is being held for investigation of a first-degree murder charge.

Police said when they arrested him, he was carrying a duffel bag, but it was empty. Police suspect he put the girl's body in the bag after she was slain and disposed of her somewhere in the Capitol Hill neighborhood.

They said he also had a bus pass, so it is possible he took a bus or light rail somewhere to get rid of the body, police said.

"We need to re-create what these two did yesterday prior to the disappearance of the 3-year-old girl," said Police Chief Gerry Whitman.

Police are going through the Denver alleys and Dumpsters, searching for 3-year-old Niveah Gallegos. They have also asked the Denver Fire Department to search the Platte River between Sixth Avenue and Interstate 70 for the girl's body.

On Saturday night, Whitman said officers were focusing the search between Sixth and 20th Avenues, from Downing to Speer.

Investigators have not said why they believe she is dead. The toddler was last seen with Montoya at an apartment on 12th Avenue and Logan Street -- an apartment that has since been sealed with crime scene tape.

Miriam Gallegos also faces a charge of filing a false report. At about 3 p.m. Friday , Gallegos told officers that she was taking Niveah to the hospital when two men she didn't know kidnapped the girl and drove off in a white vehicle. That prompted officers to issue an Amber Alert in search of the vehicle and the girl. But about an hour into the investigation, police canceled the alert and said that Niveah was the victim of foul play, but they wouldn't say what led them to believe that.

Niveah was last seen wearing a red, long-sleeve shirt with an elephant on the front, red sweat pants and pink princess shoes. She has straight brown hair, brown eyes, is about 3 feet 6 inches tall and weighs about 45 pounds.

Montoya is a registered sex offender and has been convicted of child abuse and indecent exposure to minors and adults. He has also been arrested on charges of kidnapping, according to police records.

He was arrested at about 9 p.m. near Lincoln Street and 13th Avenue. Officers said they saw a man who looked like Montoya. When they confronted him, there was a brief scuffle and the man ran off. Police chased him, took him into custody and confirmed that it was Montoya.

Gallegos also has a criminal history. In 2006, she was accused of possession of marijuana and found guilty of possessing drug paraphernalia.

Anyone who has seen Niveah is asked to call 911, the Denver Police Department at 720-913-2000 or Crime Stoppers at 720-913-STOP (7867).