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NEW YORK (CNN) -- On a July morning in 2005, Noel Miller came home from working the night shift at an Iowa nursing home. She told police she arrived around 6:20 a.m. and discovered that her 5-year-old daughter, Evelyn, was missing.

Evelyn Miller, 5, was found floating in the Cedar River a mile from her home in July 2005.

Everyone else was fast asleep.

Some 1,500 volunteers joined the six-day search for Evelyn Miller, but it ended tragically. The little girl was found floating in the Cedar River near rural Floyd, Iowa.

That was four years ago, and police still have not charged anyone in Evelyn's death.

At the time she disappeared, Evelyn was living in an apartment complex with her mother, two half-brothers and the man her mother planned to marry, Casey Frederiksen. Two friends were visiting him. They had spent the night, which was not unusual.

Noel Miller looked for her daughter, then called 911. The call launched a massive search for Evelyn, beginning at her apartment and extending outward for seven miles.

"That area was completely covered," Floyd County Sheriff Rick Lynch said. Police and volunteers hiked through cornfields and wooded areas. Authorities deployed infrared airplanes and sent dive teams into local rivers and quarries.

On the sixth evening of the search, two kayakers found Evelyn's body floating in a river just one mile from her home.

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"It takes anywhere from three to five days if it's a drowning, then the body will come to the surface," Lynch said.

The Floyd County Sheriff's Office has not released what caused Evelyn's death. But Lynch said, "We believe she was a victim of foul play."

"This is definitely a homicide," said Floyd County Attorney Jesse Marzen. "We want to make sure that law enforcement investigates as completely and as thoroughly as possible."

Investigators have narrowed their suspect list to three people, Lynch said, but police are not ready to publicly name them.

"I'm sure the general public is not going to be surprised," Lynch said, adding, "I feel confident that we will make an arrest soon."

Since the little girl's death, the three men who were staying at the apartment have been in trouble with the law.

Miller's fiancé, Frederiksen, is serving a 14-year sentence for a child pornography conviction. He recently was named in an indictment accusing him of producing methamphetamine in the apartment where he lived with Evelyn before her murder.

Frederiksen's former attorney, Jane Kelly, said he has a new lawyer, who practices in Cedar Rapids, Iowa. That attorney, Dan O'Brien, did not return a call from CNN requesting comment.

Lynch said the investigation included all registered sex offenders in the area. "All of their alibis checked out and we've ruled them out," he said.

Frederiksen and Miller have since lost custody of their two sons, who are in foster care.

Investigators said they need additional corroborating evidence to make an arrest and are waiting for the final piece of the puzzle.

They plan to use a new piece of investigative equipment that is expected to move the inquiry forward, Lynch said. "It's something that came up in the last few months."

For now, he's not discussing details. "We just can't explain what the equipment is," Lynch added, "because if we explained when we were going to do it and what the equipment was, it could expose information to a perpetrator."

Evelyn's grandmother, Linda Christie, is hopeful police soon will provide the answers she has been waiting four years to hear. She doesn't consider Evelyn's death to be a cold case.

"They work on this case at least every two weeks," she said. But, she admits, "It is a waiting game."

To protect the investigation, police aren't discussing any potential DNA evidence.

"The story never goes to black and white for us," Lynch said. "We are on it every day. It always goes to full color."

Marzen also wants the public to know that progress is being made and the investigation remains "very active."

Anyone with information that could lead to the arrest or conviction of the persons responsible for Evelyn Miller's death is asked to call the Floyd County Sheriff's Office at 641.... A $5,000 reward is