As a mystery/suspense writer myself, I want to make my books as believable as possible. I've read magazines, online, and books to learn how others write, for material to use, and to add to my knowledge.
My first book, which came out earlier this year, Midnight Hours takes place in Oklahoma City, and three of the main characters are members of the OKC police department. Much what I wrote about the police department came from my imagination and from my experiences mainly with the country sheriff's department. I realized late in the novel that I needed a resource inside the department in order to be sure the details add to the believability of the plot if and when someone who "knows" reads the book. I now know to use that source for information from the beginning of the next book in the series.
Ways to discover facts and possibilities to help with writing any type of mystery (including the sub-genres) are many.
Have a resource person in a police department. I now have one in the OKC department. If I decide to use another city or county entity, I will contact them and ask for help.
Join email groups in the crime/mystery genre. I'm a member of Sister in Crime (http://sistersincrime.org} and their Yahoo email group, as well as the crimescenewriters Yahoo email group. Many experts, including police and forensics experts, are parts of each group and answer questions posed about material needed for plots, deaths, discovery, and procedures. I started printing many pages of responses for my mystery file. I have at least three new possible ways to commit murder, investigate them, and solve them.
Read, read, read. Read mysteries of all kinds. Note which plots, characters, and details work and which don't. Analyze why or why not.
Read writing magazines and articles which cover writing different types of mysteries.
Attend conferences with mystery sessions. I have two conferences on my wish list: Scene of the Crime, held in Wichita, Kansas, and Telling Your Story hosted by Mystery author William Bernhardt, held in Tulsa, Oklahoma; but I don't know if Bill will host another one or if I miss my only chance.
The Muse Online Writing Conference the last two years had a complete list of forums, authors, and editors dealing with mystery.
The more prepared we are, the better chance our mystery will have details and information that will make our stories and novels believable.
Midnight Hours started as a novella entered in a contest on Writing.Com. The novel and long short story have a few things in common, but the novel expands and changes some of the characters and plot.
Brain Cells & Bubble Wrap