While Ernie Richardson was sworn in to his new job Monday as Mount Vernon's new fire commissioner, investigators were closing in on the man who replaced him in his former role as president of the firefighters union.

Henry Thomas, who replaced Richardson as the union leader of Local 107 on Jan. 1, resigned from the job and the department a week later.

Thomas, who is also the brother Richard Thomas, the newly elected mayor of Mount Vernon, pleaded guilty last week to federal gun trafficking charges.

According to the indictment, Thomas started importing guns from out of the country in 2010 and continued until doing so until 2015.

Authorities say they built their case by recording meetings and capturing Thomas on surveillance video.

One video, prosecutors say, shows Thomas offering to sell silenced firearms to a confidential source for more than $3,000.

At Monday's swearing-in, Richardson wouldn't comment on Thomas' resignation.

Mayor Thomas says he had no idea his brother was embroiled in the alleged criminal activity.

Robert Kelly, the new public safety commissioner, issued a statement saying, "As soon as the mayor became aware of these charges, he directed me to take whatever actions I needed to under the city's charter. Before we could act, the firefighter in question resigned."

Henry Thomas is currently out on bail and awaiting sentencing.