Federal prosecutors in Minneapolis have convened a grand jury and called witnesses to testify over the past week in connection to a civil rights investigation into Derek Chauvin, signaling a renewed push in the Justice Department's case, according to sources familiar with the secret proceedings.

The former Minneapolis police officer is preparing for a March 8 trial in state court for the killing of George Floyd.

The federal investigation into Chauvin involves a 2017 arrest in which Chauvin allegedly jammed his knee into the back of a 14-year-old boy for several minutes while ignoring his pleas that he couldn't breathe — an episode that state prosecutors said bore a striking resemblance to Chauvin's conduct in his fatal encounter with Floyd, according to the sources, who were not authorized to speak on the record. The 2017 case has been deemed inadmissible by the judge in Chauvin's upcoming trial, but was made public in court documents.

Last summer, the Justice Department promised a robust investigation into Chauvin and three other officers implicated in Floyd's death, as anti-police brutality riots tore through the Twin Cities and spread across the United States. On May 28, outside the FBI field office in Brooklyn Center, Minnesota U.S. Attorney Erica MacDonald said the federal investigation would focus on whether the officers used their authority as law enforcement agents to deprive Floyd of his constitutional rights. To meet the bar of this federal civil rights violation, investigators must prove the officers took action — or failed to do so — with knowledge of wrongdoing.

At the time, MacDonald, calling the case a top priority for the Justice Department, said the federal investigation "must be done right" and it could take time to build. The federal case has been quiet until prosecutors recently began calling witnesses before the grand jury — a private proceeding in which a group of citizens decides whether a charge is warranted — a revelation first reported by the New York Times and confirmed by Star Tribune sources Tuesday.

Chauvin was charged with several felonies, including second-degree unintentional murder and manslaughter, after bystander videos showed him kneeling on Floyd's neck for more than nine minutes last May. Former officers J. Alexander Kueng, Thomas Lane and Tou Thao are each charged with aiding and abetting second-degree murder and manslaughter.

The Justice Department investigation runs parallel to the state case, meaning it could proceed regardless of the verdicts in the upcoming state trials for Chauvin and the others.

The U.S. Attorney's Office declined to comment on the issue. A spokesman for the Minneapolis Police Department did not immediately respond to requests for comment on the proceedings.

One of the cases in Chauvin's past being examined by the grand jury occurred on Sept. 4, 2017. A description of the encounter, introduced in a motion in the state case, says Chauvin and another officer responded to a woman who said her juvenile son and daughter assaulted her. The officers found the son lying on the floor inside the house and ordered him to get up because he was under arrest. When the boy refused, Chauvin struck the teen on the head with his flashlight, then grabbed him by the throat and struck him again with the flashlight.

In November, prosecutors in the state case against Chauvin filed a memorandum to introduce the footage, which they said showed, similarly to the Floyd case, that "when faced with a suspect who does not immediately comply with his demands, Chauvin intentionally uses a level of unreasonable force to accomplish subdual and restraint."

The video also showed Chauvin applying a neck restraint to the boy, who briefly lost consciousness, before placing him in a prone position with a knee in his back for about 17 minutes until paramedics arrived, according to prosecutors. Chauvin's attorney argued that the use of force was in keeping with the department's then-policy on dealing with uncooperative suspects.

Judge Peter Cahill ruled that evidence would not be admissible in Chauvin's trial.

Staff writer Chao Xiong contributed to this report.