An Atlanta man who threw a concrete block through a city contractor's window and left dead rats on his porch pleaded guilty Tuesday to obstructing justice in a federal investigation into a pay-to-play scheme for city contracts.

Shandarrick Barnes, 41, was trying to keep construction contractor Elvin R. Mitchell Jr. from cooperating with investigators, federal prosecutors said. This was the fourth guilty plea in the ongoing investigation — Mitchell, another contractor and a former city official have also admitted wrongdoing.

Mitchell pleaded guilty in January to a conspiratorial bribery and money laundering charge. He was sentenced last month to serve five years in prison and ordered to pay more than $1.12 million in restitution. Mitchell's lawyer has said he is cooperating with investigators.

"Barnes threw the concrete block through Mitchell's dining room window to get him to 'shut up,'" U.S. Attorney Byung J. "BJay" Pak said in an emailed statement. "Instead, that violent act made Mitchell even more resolute in his cooperation with federal law enforcement."

IRS and FBI agents investigating corruption at city hall approached Mitchell in July 2015 for an interview and discussed corruption allegations and potential tax improprieties, prosecutors said. Mitchell told other people the agents were asking questions.

Mitchell told federal investigators in a Sept. 2, 2015 interview that he'd regularly paid "up-front money" for city contracts, and had another meeting with the investigators six days later.

At around 5:30 a.m. on Sept. 11, 2015, Barnes threw a concrete block with the message "ER, keep your mouth shut!" written on it through a window in Mitchell's home, prosecutors said. When Mitchell went outside, he found dead rats on his porch, on his car and in his mailbox.

Security video from Mitchell's subdivision showed a vehicle matching Barnes' leaving the area within minutes of the block crashing through the window, and investigators were able to link Barnes to the crime.

In interviews in July and August 2016, Barnes admitted to federal agents that he had thrown the block, prosecutors said. Barnes told investigators he knew about the federal investigation and that agents had asked about Mitchell's taxes and payments Mitchell made to businesses associated with Barnes' employer. He also said he knew Mitchell was cooperating with the investigation.

Barnes told investigators he was angry and that he threw the block through Mitchell's window to keep Mitchell from talking to agents about possible tax violations because he thought that could negatively affect his employer's business, prosecutors said. Prosecutors did not identify Barnes' employer.

Barnes is scheduled to be sentenced Feb. 7.

Another city construction contractor, Charles P. Richards Jr., was sentenced last month to serve two years and three months in prison and pay $193,000 in restitution. He had pleaded guilty in February to a conspiratorial bribery charge.

Prosecutors have said Mitchell paid more than $1 million and that Richards paid $193,000 to someone in exchange for city construction contracts and that they believed some of the money would be paid to one or more unnamed city officials with influence over the contracting process.

Prosecutors have not said who the pair paid.

The city's former chief procurement officer, Adam L. Smith, pleaded guilty in September to conspiratorial bribery and is set to be sentenced in January. Prosecutors say Smith accepted bribes to give contracts to an unnamed vendor.