A couple in Hawaii were arrested on suspicion of murdering their six-year-old adopted daughter, whom they recently reported missing

• The search for a missing Hawaii girl has turned into a murder investigation, Honolulu police said.
• Isabella "Ariel" Kalua's adoptive parents are now the sole suspects in her homicide.
• Police believe that the six-year-old died almost a month before she was reported missing.

The adoptive parents of a six-year-old girl were arrested Wednesday on suspicion of her murder, police in Hawaii said.

Isabella "Ariel" Kalua was missing on September 13 by Isaac and Lehua Kalua, who said they last saw her when they put her to bed the night before.

But after an extensive search, police said they think Isabella had in fact been killed as long as a month before the missing-person report, according to Ben Moszkowicz, a senior Honolulu Police officer who spoke at a press conference Wednesday.

Police have ruled out other suspects for the time being and were searching the Kaluas' home as well as the Pearl Harbor shipyard where Isaac Kalua worked, Moszkowicz said.

"Unfortunately what began as a search for a missing girl turned into a murder investigation focused on the Kaluas," said interim Police Chief Rade Vadic. No remains have been recovered and the cause of death is still unknown, he said.

When they reported her missing, the Kaluas told police that they had last seen her when they put her to bed the night before, said Moszkowicz.

But much of the information they received at the outset was false, Lieutenant Deena Thoemmes told the press conference, without elaborating on the details. This slowed the investigation, she said.

Vadic said he was unable to explain what had led police to suspect towards the Kaluas because the investigation was still live.

"At the onset there was a lot of questions," said Thoemmes, adding: "There were some things that just weren't adding up for us."

The Kaluas were initially cooperative "and then eventually they weren't," she said, saying they began not returning calls. Clues about the Kaluas' mindset, provided by the FBI's behavioral analysis unit, helped move the investigation forward, Moskowicz said.

Isabella's disappearance spurred hundreds of volunteers and HPD officers to help in the search, the officers said.

Isaac and Lehua Kalua adopted Isabella within the last two years, according to NBC News. They legally changed her name from her birth name, Ariel Sellers, but continued to refer to her as "Ariel," as did police, Moszkowicz said Wednesday.

Isabella's biological aunt, Alena Kaeo, told local outlet KHON2 that for her biological family the news "is like a bomb in our head and our faces."

"We didn't have answers and then, boom, all of a sudden they're arrested and they have the house being raided," she added.

In June, the Kaluas applied to withdraw Isabella from state school and enrol her for homeschooling, something that Kaeo said was "suspicous," KHON2 reported.

The Kaluas are in custody and cooperated in the arrest, Moszkowicz said, adding that police cannot interview them until they have lawyers. That is likely to be Friday, he said.