The Democratic National Convention issued an apology Tuesday after a bus it chartered was seen dumping raw sewage in Gwinnett County.

A DNC spokesperson called the unusual incident "an honest mistake."

"This was an honest mistake and we apologize to the Lawrenceville community for any harm we may have caused," the spokesperson said in an emailed statement. "We were unaware of any possible violations and have already taken corrective action with the charter bus company to prevent this from happening again. Furthermore, the DNC will work with the Georgia Department of Natural Resources, as well as local and state officials to determine the best course of corrective action."

The incident happened around 9:30 a.m. in front of the O'Reilly Auto Parts store at 525 Grayson Highway in Lawrenceville.

An employee of the store, Mike Robins, told Lawrenceville police he saw someone get out of the bus —  which bears the slogan "Forward Together" and the likenesses of presidential candidate Hillary Clinton and running mate Tim Kaine — and dump "its sewage into the storm drain," according to an incident report.

Gwinnett County fire hazmat crews also responded to the scene and cleaned up the spill, the police report said. Lawrenceville police spokesman Capt. Greg Vaughn said the investigation has been turned over to Georgia's Environmental Protection Division.

The bus kicked off a nationwide tour Tuesday with stops in Lawrenceville, Atlanta and Macon.

"Georgia has become increasingly competitive for Democrats this election cycle," a press release announcing the tour said, "and the Democratic Party is campaigning in Georgia to provide additional support for Hillary Clinton and to promote Democrats up and down the ballot."