ST. LOUIS • Police have arrested a 50-year-old man in connection with what appears to be a firebombing that caused minor damage at the campaign headquarters of U.S. Rep. Russ Carnahan, D-St. Louis.

Firefighters responded after witnesses called about hearing broken glass and later smelling smoke at the office in the 7000 block of Chippewa Street in south St. Louis.

At the office it later appeared as if someone had thrown a burning object through a window.

The incident took place around 2:20 a.m. Tuesday and is being investigated as an arson by the police department’s bomb and arson squad.

Police have not released the name of the suspect, who they described as a white man, pending the application of warrants. They also did not indicate a potential motive.

The Carnahan campaign issued a brief statement confirming the incident and minor damage. On Tuesday evening, the room, which the campaign uses for its finance operation, still smelled of smoke with charred campaign papers on a table.

The building is marked with Carnahan campaign signs. Carnahan, first elected in 2004, is locked in a tough election battle with Republican Ed Martin.

Although the congressional seat had been considered reliably Democratic — it was held for many years by former U.S. Rep. Dick Gephardt, a one-time Democratic presidential contender — Carnahan is facing a tough challenge this year. Martin has kept pace on fundraising, and he has criticized Carnahan on a range of issues.

Apart from Martin, Carnahan is weathering a tumultuous political environment. A year ago, several people were arrested at a Carnahan town hall forum after a fracas broke out between protesters and Carnahan supporters.