A Delta Air Lines flight approaching Chicago’s Midway International Airport was hit by a fireworks mortar during the July 4 celebrations, prompting the crew to report a sudden “big bang” as the aircraft descended.

According to accounts relayed through air-traffic communications, the crew initially suspected something like a mortar went off below them. The aircraft then landed normally and taxied to the gate. Delta said there were no injuries among passengers or crew, and that the incident caused minor damage.

Chicago police said they were investigating and connected the case to the broader federal inquiry, noting the need to determine the origin of the fireworks and whether anyone violated local or federal rules governing the discharge of fireworks.

The report also notes the timing: the incident occurred amid nationwide public and private fireworks displays tied to the United States’ Independence Day celebrations.