If your novel features law enforcement officers, consider the size and responsibilities of their organization. The size will determine the capabilities of the agency and its officers. Ninety percent of the law enforcement agencies in the U.S. have less than 25 officers. A department with less than 100 officers probably does not have full-time "CSI" technicians nor a full-time SWAT team. Those are usually secondary duties for patrol officers and detectives in addition to their primary responsibilities. Most detectives are going to process their own crime scenes--"CSI" units only exist in large departments and extremely well financed ones.
A common mistake in mysteries and suspense novels is using the FBI to investigate the murder. The FBI does not routinely investigate murders--the rare exception would be a violation of federal law. The FBI does not have the authority to take over murder cases. Not even serial killings. 99.9% of all U.S. murders are investigated by local police. In 40+ years in law enforcement, maybe a half dozen of the thousands of cases and arrests I participated in also involved the FBI. It happens more often in fiction than real life. The FBI has its own federal investigations to worry about.
Now, if you need a character to pair up with a local detective, use a county sheriff's deputy, state police investigator, or whatever state enforcement agency exists in your state. While that's also rare, it occurs occasionally. The odds your local cop will partner up with a fed is slim to none. It just doesn't happen.