Accused Long Island serial killer Rex Heuermann has been indicted on a new murder charge in the killing of a fourth woman, Maureen Brainard-Barnes, who investigators believe was the first of the “Gilgo Four” to be killed, according to an indictment unsealed Tuesday.

The unsealing of the indictment came after authorities announced Heuermann would face “a major development” in the investigation of the Gilgo Beach killings, which all victimized sex workers and lay unsolved for more than a decade.

Related: Two sex workers call Gilgo Beach serial killer suspect ‘violent’ and ‘aggressive’

Heuermann’s indictment was handed by a special grand jury that was convened to consider an indictment against the Manhattan architect in the killing of Brainard-Barnes, whose remains were found close to three other women in marshland along Gilgo Beach.

Heuermann, 60, had been named as the “prime suspect” in Brainard-Barnes’s disappearance. In July, he was charged with first- and second-degree murder in the killings of the three other women – Melissa Barthelemy, Megan Waterman and Amber Lynn Costello. According to prosecutors, investigators linked Heuermann to the murders through DNA, cellphone site data and burner phones.

In all, 10 sets of remains – all of sex workers – were discovered in the area, with police theorizing that all died at the hands of one or more serial killers.

Brainard-Barnes was 25 years old when she went missing on 9 July 2007 after calling a friend to say she was be going to meet someone outside a Manhattan motel on an “out call”. Three years later, on 13 December 2010, her remains were found near Gilgo Beach, during the search for Shannan Gilbert, whose death in nearby marshes has never been fully explained.

Brainard-Barnes was also the victim whose remains were bound with three perished belts, one with a distinctive buckle with the embossed initials “WH” or “HM” that investigators publicly linked to her killer and used four years ago to stir interest in what was then a cold case.

The initial three of Heuermann’s alleged victims were linked to the accused man using mitochondrial DNA from a pizza crust and a used napkin that were allegedly discarded by Heuermann near his Manhattan office. It was then linked to a hair found on green burlap sacking used to “restrain and transport” the remains of one of the victims.

Heuermann’s attorney, Michael Brown, has said his client denied the accusations against him. The case was broken open last year when investigators linked a distinctive Chevrolet pickup described by a friend of Amber Costello’s to the suspect.

Ahead of an upcoming trial, Brown has said that the DNA claims presented by prosecutors may only place Heuermann in a pool of “thousands and thousands” of possible matches.

But the case continues to roil Long Island law enforcement, amid allegations of longstanding corruption that stalled the investigation. Last year, the Suffolk county police commissioner, Rodney Harrison, who led the taskforce into the 14-year-old case, abruptly stepped down.

It was later claimed that Harrison had changed vacation days to sick days on previously filed timesheets. But others speculated that Harrison had gotten too close to John Ray, an attorney representing the family of Shannan Gilbert who maintains that police corruption had hampered the Gilgo investigations.

In October, soon after Harrison attended a press conference convened by Ray to further his claims, local district attorney Ray Tierney issued a slap-down, saying that his office would “continue to investigate this case through the Grand Jury process” and “not through press conferences”.

“Any attorneys representing victims or their families, by definition, have a conflict of interest and should not be a part of the investigation,” Tierney added.