DENVER (Reuters) - Human remains unearthed by an excavation crew at an oil and gas site in Colorado this week are those of a 12-year-old girl who went missing over three decades ago, but the mystery surrounding her fate has yet to be solved, police said on Thursday.

Jonelle Matthews vanished in December 1984 after she was dropped off at her home in Greeley, Colorado, about 50 miles (80 kms) north of Denver, following a performance singing in a school Christmas concert with her classmates.

On Tuesday afternoon, a crew working on a pipeline in rural Weld County uncovered what appeared to be human bones and notified authorities, the Greeley Police Department said in a statement.

“Based on the evidence found on scene and the coroner’s examination of the remains, an identification ... has been made,” the statement said. “The remains are identified as Jonelle Matthews.”

Police did not say whether the cause or manner of the girl’s death had been determined, and offered no further details except to say the investigation into what happened to the seventh-grader “remains active.”

On the 34th anniversary of her disappearance in December, police said in a statement that they were taking a fresh look at the case, including efforts to interview witnesses and anyone who may have had contact with the girl in the weeks before she disappeared.

“Given the significant recent advances in forensic and scientific testing methods, Greeley Police will be examining every possible forensic option available to us,” police said at the time.

Police said on Thursday that they were “relieved” to finally get a break in the case.

“This case has weighed on the hearts of the Greeley Police Department, the family and the entire city,” the police statement said.