Missoula police recovered a handgun they believe was used in a foot chase off West Broadway in Missoula.
Twenty-year-old Joseph Peasley of Missoula was arrested for violating probation.
Police said he is a violent offender.
The officer who arrested him is on paid leave, pending the completion of an in-house investigation, after firing his weapon.
A search and rescue crew floated a pontoon into a waterway feeding into the Clark Fork River Monday morning and found the gun investigators believe Peasley dropped.
It was in three feet of water.
"It was a 40-caliber semi-automatic handgun," said Sgt. Scott Pastian, "which was loaded with ammunition."
Police took the gun and ammunition into evidence.
Searchers did not find the bullet casing from the gun patrol officer Ben Slater fired.
"It was a warning shot toward the bank," said Pastian.
Reports said Slater believed Peasley was reaching into his waist band, as if for a weapon.
His warning shot stopped the suspect, who was then arrested.
On Sunday, Missoula police answered a 911 dispatch call from someone who said Peasley was on his way to their home with a weapon.
They said Slater discovered the suspect on a bicycle, but he refused to stop.
Police said Peasley abandoned the bike and started running toward the river.
It happened in the parking lot of an apartment complex.
"It's scary," said neighborhood resident Paul Tappon, "anytime there's a bullet flying around in a residential area."
Missoula's city police manual says warning shots may be fired if an officer is authorized to use deadly force, and only if the officer reasonably believes a warning shot can be fired safely.