According the Officer Down Memorial Page, there were 126 law enforcement officers killed in the line of duty in 2012. That inflates things a bit, because 18 of them were police dogs.
8 police dogs died of heat exhaustion
2 K9s killed in transportation related incidents.
6 K9s were shot
1 died from a fall
Let's take a look at what happened to the 108 human cops, shall we?
Of those 108:
There were 42 officers killed in transportation incidents, 39% of those killed.
3 from aircraft, 35 in auto accidents, 5 were struck by vehicles as pedestrians.
16 of them (15%) died from medical problems, or from accidents:
2 were killed by unspecified "medical emergencies"
1 died from hepatitis C that he contracted in 1983
1 died from complications of a surgery to repair an on duty knee injury
2 cops were killed by falling from heights
1 died from injuries sustained in 2009 when he fell off of a horse
1 died from heat exhaustion
7 died from heart attacks
1 killed in a training accident when an instructor threw him and caused a head injury
Of the 17 cops killed by non-firearm assaults:
11 were run down by criminals
1 was killed with an assailant's bare hands
5 were stabbed. All 5 were stabbed by jail or prison inmates.
Of the 43 firearm deaths:
1 was killed by her husband, using her own gun
1 was killed by an assailant who overpowered the officer and shot him with his own gun
2 were shot by other cops in cases of mistaken identity
2 were shot and killed when a prisoner that was in custody, was handcuffed in a patrol car, and had already been searched, produced a handgun and shot them.
1 was ambushed and killed in his driveway by a convicted felon that he had arrested many times over a 40 year career.
1 was shot in 1965 and died in 2012. Hard to say this death is due to being shot when you live another 48 years.
2 were ambushed by domestic terrorists while working off duty jobs in their patrol cars.
By weapon type
19 were shot by unknown firearms
18 with handguns
1 with a shotgun
3 with unknown type rifles
4 with so called "assault weapons"
So out of the 108 cops killed in 2012, there were 36 cops killed by assaults on the street by people that they came in contact with. The rest of the deaths were accidents, medical problems, being shot by other cops, by their spouses, and other incidents. It is a myth that cops have a job that is so dangerous as to require that they disarm and subjugate every citizen that they come into contact with for "officer safety."
If they really cared about officer safety, they would take away their lights and sirens, and force them to drive at reasonable speeds. That would have saved the lives of the same number of officers, and not led to violating people's rights and roughing them up.