Taken from bradycampaign.org FAQs section (though I think some data is inaccurate):
Q: Does law enforcement support the ban on assault weapons?
A: Every major national law enforcement organization in the country supported the federal assault weapons ban and worked for its passage. Among the many law enforcement organization that supported the ban are the Law Enforcement Steering Committee, the Fraternal Order of Police, the National Sheriffs' Association, the International Association of Chiefs of Police, the Major City Chiefs Association, the International Brotherhood of Police Officers, the National Association of Police Organizations, the Hispanic American Police Command Officers Association, the National Black Police Association, the National Organization of Black Law Enforcement Executives, the Police Executive Research Forum, and the Police Foundation.
Q: Why did police support the ban so strongly?
A: While there are no exact numbers of assault weapon incidents, police across America in the 1980s reported that semi-automatic assault weapons had become the "weapon of choice" for drug traffickers, gangs and paramilitary extremist groups.
Law enforcement officers are at particular risk from these weapons because of their high firepower and ability to penetrate body armor. In addition, limiting civilian access to such weapons lessens the need for law enforcement to carry assault weapons themselves in order to match the firepower capability that criminals with assault weapons would have. Law enforcement officers do not want to have to carry M-16s as their standard service weapon. In 1997, after a North Hollywood, CA shootout in which police were outgunned by two men with assault weapons, Jim Pasco, executive director of the Fraternal Order of Police stated
An AK-47 fires a military round. In a conventional home with dry-wall walls, I wouldn't be surprised if it went through six of them...Police are armed with weapons that are effective with criminals in line of sight. They don't want and don't need weapons that would harm innocent bystanders.[2]
Ray Kelly, the Treasury Department's undersecretary for enforcement at the time, noted that police departments have specially trained officers who use high-powered weapons. "It takes a lot of training to be proficient at it," he said. "I don't think you can issue high-powered weapons to every patrol officer."[3]
Prior to the ban's passage, assault rifles were used to kill and injure dozens of innocent people in some particularly heinous crimes, including:
I BELIEVE MORE DEATHS OCCUR FROM SMALL CAPACITY WEAPONS MYSELF
The Stockton schoolyard massacre - On January 17, 1989, Patrick Purdy killed 5 small children, and wounded 29 others and 1 teacher at the Cleveland Elementary School in Stockton, California, using a semi-automatic version of the AK-47 assault rifle imported from China. That weapon had been purchased from a gun dealer in Oregon and was equipped with a 75-round "drum" magazine. Purdy shot 106 rounds in less than 2 minutes.[4]
The San Francisco Pettit & Martin shootings - On July 1, 1993, Gian Luigi Ferri killed 8 people and wounded 6 others at the San Francisco law offices of Pettit & Martin and other offices at 101 California Street. Ferri used two TEC-DC9 assault pistols with 50-round magazines. These weapons had been purchased from a pawnshop and a gun show in Nevada.[5]
The CIA headquarters shootings - On January 25, 1993, Pakistani national Mir Aimal Kasi killed 2 CIA employees and wounded 3 others outside the entrance to CIA headquarters in Langley, VA. Kasi used a Chinese-made semi-automatic AK-47 assault rifle equipped with a 30-round magazine, purchased from a Northern Virginia gun store.[6]
The Branch-Davidian standoff in Waco, Texas - On February 28, 1993, while attempting to serve federal search and arrest warrants at the Branch-Davidian compound in Waco, Texas, four ATF special agents were killed and 16 others were wounded with an arsenal of assault weapons. According to a federal affidavit, the cult had accumulated at least the following assault weapons: 123 AR-15s, 44 AK-47s, 2 Barrett .50 calibers, 2 Street Sweepers, an unknown number of MAC-10 and MAC-11s, 20 100-round drum magazines, and 260 large-capacity banana clips. The weapons were bought legally from gun dealers and at gun shows.[7]