Boise, Idaho ? The 30-year-old mother of three jumped from her disabled SUV following a chase, holding a gun to her head to keep police back. Officers fired a stun gun but the nonlethal weapon was foiled by her heavy coat.

When she pointed her handgun at the two nearest deputies, officers switched to assault rifles, hitting Sarah Marie Stanfield of Boise eight times with bullets designed to break apart on impact to increase internal damage. She died last fall of multiple gunshot wounds.

Some jurisdictions across the U.S. have been arming rank-and-file officers with high-powered assault rifles for a decade or more. But law enforcement officials say that trend has accelerated in the last year because of greater numbers of shootouts, standoffs in which police were outgunned, rising officer deaths and mass shootings of civilians by heavily armed gunmen.

Only patchwork information is available on how many law enforcement agencies are outfitting deputies and patrol officers with assault rifles. But from the Midwest to Miami to college campuses, agencies are acquiring AR-15s or M-4s, both close relatives of the military’s M-16. The rifles fire bullets with enough velocity to penetrate some types of body armor and have greater accuracy at longer range than handguns.

Last year, Miami Police Chief John Timoney authorized his patrol officers to carry AR-15s because of a rise in assault rifle use by criminals.

“This is a national problem. Police agencies all over the U.S. are going to bigger weapons,” said Timoney, whose agency now has about 50 AR-15s and expects to get 150 more. He blames the 2004 expiration of the federal ban on assault weapons for the escalation of heavily armed violence.

In 2007, according to preliminary numbers compiled by the National Law Enforcement Officers Memorial Fund, 69 officers were shot to death, up from 52 in 2006 and the most in five years, fund spokesman Kevin Morison said.

“There just seems to be more brazen, cold-blooded killers out there,” he said.