The Los Angeles Police Department bicycle coordination unit is running a pilot program that put officers on electric bikes. They're pretty schmancy bikes, too. They can go up to 20 mph, top speed for typical cyclist. The manufacturer donated the bikes to the police department, but they retail for $3,499.
The program began because LAPD officers couldn't keep up with road bikes on the mountain bikes they already own. The electric bikes may not be quite as intimidating as those horses police departments trot out sometimes, but they do give officers an edge over misbehaving cyclists who only have their legs to power their getaway. It's unclear whether the bikes are equipped with sirens, though. Maybe just an extra loud bike bell.
LAPD experiments with electric bikes, Los Angeles Times.