Federal authorities are investigating the conduct and oversight of a firearms analyst for the District’s Department of Forensic Sciences, an agency that has come under scrutiny in two other instances for problems with its handling of crime scene evidence.

The investigation involves an allegation that the examiner falsely indicated that his analysis of evidence had been verified by a colleague, when it actually had not undergone a required review, according to one person familiar with the investigation. When a complaint was made, a supervisor allegedly instructed the colleague — whose name had essentially been forged — to play down his concerns.

The independent city forensics lab, which opened in 2012 in a $220 million facility in Southwest Washington, handles the examinations of DNA, ballistics and other evidence collected in hundreds of criminal cases. Authorities rely on the work of forensic analysts to build criminal cases, and analysts may be called on to testify in court.

In October, the Justice Department asked the D.C. department to turn over documents and communications dealing with firearm evidence examination policies as well as any reviews or indications of errors, according to a document obtained by The Washington Post. The request called for material dating to 2015.

It was unclear whether any errors have been uncovered in firearm analysis reports.

Two people who described the investigation to The Post spoke on the condition of anonymity because it is ongoing.

Spokesmen for the FBI and the U.S. attorney’s office in the District declined to confirm the investigation. A spokesman for the Department of Forensic Sciences did not answer questions regarding the investigation.

Part of the investigation includes an audio recording that allegedly captured the supervisor telling one analyst to change his wording on a document that described his concerns about a colleague’s conduct, according to a person familiar with the recording.

The investigation comes as city officials have been working feverishly to stem increasing gun violence in the city. Last year, D.C. police reported 166 homicides, the highest death toll in a decade. Prosecutors announced they would pursue more gun possession cases in federal court, where defendants could face tougher sentences.

Firearms examiners study guns, bullets, shell casings and other forensic evidence, sometimes linking a particular weapon to a crime. As part of the lab’s quality control, their forensic reports are then reviewed by colleagues to ensure there were no errors in the examiner’s findings.

Publicly, the firearms lab has been celebrated as essential to the city’s ongoing efforts to reduce crime. In September, District officials including Mayor Muriel E. Bowser (D) and D.C. police top brass gathered at the Department of Forensic Sciences with the department’s director, Jenifer Smith, to announce the expansion of the Firearms Examination Unit, including the hiring of four additional examiners.

Former lab employees said in interviews that they had felt pressured to quickly conduct examinations during a time when the lab was experiencing high employee turnover. In addition, working relationships between prosecutors and examiners at the lab have been strained at times over disagreements about how analyses are conducted and the timeliness of when they are completed.

In a February 2019 report to the City Council, the department’s officials said the agency was working to reduce a backlog of firearm examination cases. According to the report, in January 2019, there was a backlog of 474 cases, down from 1,089 in January 2018.

FBI agents and federal prosecutors have been interviewing current and former employees of the firearms lab, according to four individuals familiar with the investigation. Two former employees said they were instructed by the FBI to sign nondisclosure agreements. Kelsey Pietranton, an FBI spokeswoman, declined to comment on such practices.

The written request by the Justice Department also sought all firearm examinations performed by examiner Steven Chase since 2015. Repeated voice-mail messages left for Chase were not returned.

The lab has had other problems since it opened in 2012.

In 2017, federal prosecutors alerted D.C. defense attorneys that crime lab officials had to review more than 150 firearm examinations for accuracy. Officials at the time said one examiner made mistakes on two cases, and two other examiners incorrectly confirmed the erroneous findings. Lab supervisors at that time said they reviewed cases of the three examiners dating back to 2015, which is the same time period for the current investigation.

In 2015, federal prosecutors said there were errors in the way analysts determined whether a sample could be linked to a suspect or a victim. The lab’s DNA testing was temporarily halted and its director replaced.

The new federal investigation also comes as officials in Maryland are looking into the work of a state gun analyst who testified in hundreds of cases.

Joseph Kopera, who was head of the Maryland State Police firearms unit, initially came under scrutiny in 2007 when it was revealed that he lied about his credentials. He died of a self-inflicted gunshot wound the same year.

Then, in fall 2019, state police officials discovered that on some reports Kopera had forged the initials of a co-worker who ostensibly reviewed and verified his work. That finding prompted authorities to launch a review of more than 4,000 of his case files. Whether the review of those Maryland cases will result in overturned convictions or new trials is unclear.

Jennifer Jenkins and Julie Schmidt contributed to this report.