Residents say a mysterious smell — one that has lingered for months in past years — is back in downtown Hattiesburg.

The Hattiesburg American reports that its source is unknown.

The smell has also been reported in areas surrounding downtown, including Petal.

Some residents say the smell is like ammonia, with others comparing it to the scent of a rotting animal corpse.

The smell is typically most noticeable in the morning, but sometimes rolls in during the evening hours as well, the Hattiesburg newspaper reported.

Hattiesburg Mayor Toby Barker said city officials will continue in their quest to track down the source.

“Finding the cause might take some time, so we ask your patience as we use every resource available to us through the Mississippi Department of Environmental Quality,” Barker said in a recent update on Facebook.

The city has been working with the Mississippi Department of Environmental Quality since December to determine the cause, Barker said.

The mayor says testing found the city’s sewer lagoons to be working properly. The lagoons are tested multiple times per month to see whether they are operating within limits of the city’s state permit. The city hasn’t had a violation at its lagoons since 2017, the newspaper reported.

The city has also reached out to several industrial employers to determine a potential source.