You should check out some info on "police personality":
http://www.abuseofpower.info/Culture_Brotherhood.htm While there are many non-corrupt cops, there are many who feel they have a special authority over others. People with certain personalities are drawn to police work and many police have no problems lying if they feel the ends justify the means. I have had a cop blatantly lie on a police report. Not only that, he wrote down the witnesses' name, address, AND phone number all slightly incorrect so no one could contact the witness. We literally had to use a separate party to hunt down the witness who then signed a sworn affidavit that the police officer blatantly lied about his statements and lied on the police report. It was an endless investigation, that got the police report changed three times, but nothing ever happened to the officer. He was heavily protected by the police department and it took over a month and us tracking down the witness before they would even consider our complaint. Once having considered our complaint an investigator chosen BY THE POLICE agreed that his police report was not mathematically possible for the vehicles to have done what he claimed they did. They changed the police report to find the other person at fault. However, the police officer then complained so two weeks later they changed it again to "fault cannot be determined" despite no new evidence. Despite the eye witness saying that she pulled out in front of me, despite the police department's investigator saying she had to have pulled out in front of me, and despite the woman's story changing more than once. Nothing happened to the officer who obviously knew the woman in the accident. He was completely protected by the police department.
I had another officer pull me over after leaving a ball game at the high school when I was younger. He insisted he "smelled pot" in my car (even though I have never done pot or had pot in my car. I don't even drink alcohol or caffeine for that matter) and he claimed this was "reasonable cause" to search the car. It was obvious he was lying. He even did a fake over the top "sniff" thing with his nose. It took an hour of them searching my car, searching me, threatening to go to my house and search my house if I did tell them the names of people I knew who smoked pot. WHICH WAS NOBODY (though that changed when I went to college, but damn, wtf?) and the frisked my body twice, and my hat. This all happened right at the exit from the high school, where the game was letting out, where everyone saw this happening to me. Finally they found nothing and let me go, but it was a nightmare being threatened, I wanted to flat out cry. I'm a small person, 5'5 and I may be male but I'm gay and did not feel very much like I could defend myself verbally or physically if need be.
So I can say from my anecdotal evidence as well as from a good deal of research (I now have a degree in psychology from UNC-Greensboro) that police cannot always be trusted to tell the truth on a police report. I can provide links to plenty of research indicating a very specific type of personality that serves in the police force. The sad thing is, before these two experiences, I used to think cops were alright. And still to this day, one of my very best long-time friends is a correctional officer.
I remember learning in my Abnormal Psychology course that people who are BULLIES in school have a higher likelihood of growing up to be cops than those who are not.