FBI agents looking for 6-year-old Isabella Kalua dug up her adoptive parents' back yard hoping to find her remains

• FBI and police dug up the yard of the couple charged with killing six-year-old Isabella Kalua.
• Lehua and Isaac Kalua were charged Wednesday for the murder of Isabella, their adoptive child.

FBI agents continued their search for the remains of Isabella Kalua Thursday by digging up the backyard of her adoptive parents, who have been charged for her murder.

Isaac and Lehua Kalua were charged Wednesday with second-degree murder in her death, police in Honolulu, Hawaii, told a press conference that day.

The couple reported Isabella missing on September 13. But investigators now believe the girl was dead around a month before that time, according to Ben Moszkowicz, a senior officer with the Honolulu Police Department.

They are due in court on Friday.

"We believe that the evidence leads to the Kaluas and no-one else," interim Police Chief Rade Vanic said Wednesday.

At the time of the arrest, police also served a search warrant at the couple's home in the Waimanalo district of Honolulu, as well as Isaac Kalua's workplace at Pearl Harbor.

FBI agents with pickaxes, probes and markers began searching the yard as of around 2pm Wednesday, according to the Honolulu Star-Advertiser.

Investigators also opened and photographed manholes on the intersection of nearby Kakaina and Hihimanu streets, the paper reported.

Experts have said that drones with thermal imaging cameras, sniffer dogs and soil samples could also help investigators find Isabella's remains, according to local channel KHON2.

Computer records and social media traces would likely be important to the investigation, Sheryl Sunia, Criminal Justice Program Chair at Hawaii Pacific University, told the outlet.

Isabella, whose birth name was Ariel Sellers, would have turned seven years old on November 6, according to the Honolulu Star-Advertiser.

Her missing-person case inspired hundreds of people to volunteer in the search. Following the announcement of the Kaluas' arrests, community members gathered in a vigil near her home, Hawaii News Now reported.