Angels & Monsters Part 1

I have an issue with this episode and part 2. The murder of these 2 girls were committed with a Glock 40 police issued. In this episode the investigator states that local law enforcement willing brought in their weapons to be check against ballistics. 1st, I have never heard of police departments giving up their weapons to be checked. In part 2, a timeline is established tracing the weapon from Glock to the Baltimore police dept. and they were able to get the serial number to confirm the weapon was used in the crime. However, the episode glosses over the fact that 3 police officers in Oklahoma were in possession of that weapon before it ended up in the hands of Kevin Sweat. How did he get that weapon? Did someone with the local police dept. know he had that weapon at the time of the killing? A Glock 40, I would think, is an uncommon weapon used in rural Oklahoma. Did someone within the police dept. not report knowing that Kevin Sweat has such a weapon. The reason that this bothers me is that if someone sweep this under the rug because they sold the gun, under the table, to Kevin Sweat they would not want to be associated with him. Kevin Sweat was able to avoid detection and ended up killing another girl Ashley Taylor 3 years later. Did Ashley Taylor die because of someone's  inaction?