Elisa Baker, the stepmother of murdered Australian girl Zahra, has been charged with bigamy.

Baker, who has been in a North Carolina jail since Zahra went missing last year, illegally married Zahra's father, Adam Baker, in Queensland in 2008, US prosecutors allege.

Baker was still married to North Carolina man Aaron Young and did not get divorced until August 21, 2009, it is alleged.

Mr Baker, a former Queensland sugar mill worker, met Baker, an American, on the internet and apparently was clueless she remained married.

When Mr Baker and Wagga Wagga-born Zahra, 10, moved from Australia to North Carolina to be with her, prosecutors allege Baker said Mr Young was her brother.

Authorities are yet to charge anyone with the murder of Zahra, whose body was dismembered and parts tossed or buried in bushland outside the town of Hickory, North Carolina, where she lived with her father and Baker.

Zahra's prosthetic leg was also dumped in bushland.

The girl was reported missing from the family home on October 9 and Elisa Baker was arrested a day later, accused of writing a fake ransom note.

According to search warrants filed by police investigating Zahra's death, Mr Baker, Elisa Baker and Mr Young may have been involved in a bizarre "chainsaw massacre role-playing" game on the internet just days before Zahra went missing.

Briefcase found

Investigators continue to follow leads in the case, with the latest the discovery of a briefcase in bushland near where Zahra's prosthetic leg and a piece of bone were found in October.

Logging crews were working in the area on Tuesday when they found the briefcase.

The crews called police and a mouldy blanket was found inside the briefcase. It has been sent to a laboratory for testing.

The story of Zahra, who had part of her left leg amputated five years ago after a struggle with bone cancer, has made headlines around the world.

- AAP