Federal investigators are looking into an incident in which two bottles of alcohol with lit paper-towel fuses were thrown at a U.S. congressman's office early Thursday in Kansas City, breaking a window but failing to ignite before falling harmlessly outside the building.

The bottles used in the attempted firebombing — one rum and one Jagermeister — were found on the ground after police were called to Emanuel Cleaver's office around 2:50 a.m., Kansas City Police Sgt. Kari Thompson said.

Paper towels that were stuffed into the necks of the bottles appear to have been lit but went out while still in the air, Thompson said.

"This is the second incident within the last six years," John Jones, Cleaver's chief of staff, said in a statement. "The Kansas City Police have completed their initial survey of the scene and we await their report."

Jones didn't go into detail about the other incident, but Cleaver's press secretary, Mary Petrovic, said there was an incident in October 2011 in which two people threw rocks and broke the windows of the same Kansas City office building. Nobody got hurt and no suspects were ever identified, she said.

Cleaver is in Washington, D.C., and there were no staff members in the building at the time of Thursday's incident, Jones said.

The Kansas City Police Department's bomb and arson squad was called to the scene, and the FBI now has taken over the investigation.

Cleaver said in a statement that he was focused on the 13th anniversary of the 9/11 attacks instead of what happened at his office.

"Today, our thoughts should not dwell on a small incident that happened and hurt no one," Cleaver said. "Our thoughts and prayers should be with the families and friends of those killed on September 11, 2001, and all those who suffered in its aftermath."