By Barton Lorimor

SPRINGFIELD, Ill (Reuters) - The phrase "naked gun" is no longer just a movie reference here in southwestern Illinois.

Officers responding to a report of shots fired early Friday morning in nearby Cahokia quickly realized the anonymous tipster who made the call wasn't kidding: the suspected gunman wasn't wearing a single thread of clothing.

Sergeant Ken Schrader said the man, who remains unidentified, and another male fled the scene in a Chevrolet Impala SS shortly after police responded.

Officers pursued the suspect vehicle north on Illinois Route 3 at speeds over 100 mph, Schrader said.

The pursuit was called off after the vehicle was seen crossing the Mississippi River via the Poplar Street Bridge, which leads to downtown St. Louis.

A spokeswoman for the St. Louis Metropolitan Police said via e-mail Friday the department did not have any information about the incident.

Cahokia Police are continuing their investigation of the incident, hoping to understand why the suspect, described as a black male, was nude and firing a handgun into the air outside an apartment just after midnight.

Schrader said the incident is not the department's top priority, however, as officers are also investigating an unrelated bank robbery that occurred Monday afternoon. He said he did not know if this summer's excessive heat or poor economy has affected crime in Cahokia.

"But it's certainly not something you hear about every day," he said before chuckling.

(Editing by James B. Kelleher and Jerry Norton)