South Australian police say a black dress could hold the key to solving the mystery of the murder of a little girl whose body was found in a suitcase dumped by a highway.

The young girl’s skeletal remains were found dumped by the Karoonda Highway, around 2km west of Wynarka, last month alongside clothes and personal effects.

Among them was a black Cotton On For Kids dress, one of just 28 sold in South Australia between 2007 and 2010.

Superintendent Des Bray said he remained confident someone must know who the dress belongs to.

“The black dress was one of a number of items found with the little girl when we found her body,” Supt Bray told the TODAY Show this morning.

“We think somebody who [purchased] this dress must have given it to somebody that they love or other people must have seen little girls in that dress.

“The dress holds the key to it.”

A distinctive quilt found with girl's skeletal remains. (SA Police)

The murdered girl was aged between two-and-a-half and four, had fair hair and may have been killed as long ago as 2007.

Supt Bray said his team had scoured missing persons cases and ruled out more than 20 potential victims.

“This is bizarre,” he said.

“If you’d told me three weeks ago it was possible in Australia to have a little girl who’d died and nobody knows where she came from, I would have thought that was impossible.”

The suitcase in which the girl's body was found. (SA Police)

Detectives have begun an extensive doorknocking campaign, which has been widened to include homes and businesses within hundreds of square kilometres of the grisly find.

Police are also still trying to find a man, aged about 60, who was seen in the area carrying a suitcase on April 13 and again on May 26.