Remember that you can turn off the identifier in your cell phone and simply call 911 to give an anonymous tip about a suspicious activity or a crime in progress. The cops don't like the anonymity but they will respond. If they don't, call them again. Both their department recording device and your own cell phone record the date, time and the number of the call. Those records can be used to launch investigations into whether they responded correctly and promptly.
Kenya police: 2 Kenyans arrested for spying for Iran (AP)
from the article: U.S. Homeland Security asks Jewish groups to be vigilant for terrorists