In many cases an arrest warrant is not needed. If a police officer personally sees someone committing a crime, the officer may arrest that individual without a warrant. An officer may also arrest someone if they have probable cause to believe the person had committed or were about to commit a crime. For example, a police officer is notified a bank has just been robbed and is given a description of the suspect. The single robber left in a dark blue Ford Mustang. The officer stops a dark blue Ford Mustang with bags of money on the back seat (in plain sight) and the driver matches the description. The officer believes he has probable cause and arrests the driver. 