Sunday's slaying of two Las Vegas policemen raises to 23 the number of law enforcement officers killed by gunfire this year, a 53 percent increase over the tally at this time last year, which is spurring concern about the influence of radical groups.

After shooting the policemen at point-blank range, the husband and wife killers took the officers' weapons and reportedly covered them with a Revolutionary War-era Gadsden flag, which depicts a coiled snake and the words "Don't Tread on Me." The officers' killers — Jerad Miller and his wife, Amanda — had been at Cliven Bundy's ranch during his standoff with federal officials in April, though the rancher's family said Monday the two had been chased away.