First, you need to be aware that when you begin investigating your ancestors, you may discover some things that you won't be totally proud of. If you aren't sure you can live with that, you may want to walk away in blissful ignorance.
Second, although you provided very little information about your Baltimore Police Force ancestors, they are not particularly difficult to trace. For example, Sgt. Louis Zehner, who was born about 1854 in Maryland of parents from Hesse, Darmstadt, actually joined the Baltimore City Police Force before 1900 and was still serving it in 1930 at the age of 76. Pretty impressive. At that time, he was living with his second wife, Laura, at 2313 Oliver Street in Baltimore.
Officer W. O. Zehner's name was William O. Zehner, b ca 1877 in MD. He was still serving as an Officer in 1920 when he was listed as a 41-year-old lodger at 1503 North Luzerne Street in Baltimore.