What do the FBI and police cops think about private investigators? Do they work together or not? Are private investigators really legal?*

FBI Agents and Police Detectives work for the state. If the state falls short of investigation capacity, it hires Private Investigators. In that capacity, FBI Agents and Police Detectives probably work well together with PIs.

That said, it’s more common for PIs to work for the defence, studying crime scenes, following up on clues, interviewing witnesses, and so on. In that case, FBI / Police will have an adversarial relationship with PIs. But, since many PIs are ex-FBI Agents / Police Detectives themselves, I do expect a certain degree of camaraderie to exist between them.

I’m not licensed to provide legal advice but, I’m guessing that Private Investigators holding Private Investigator license – in the states / cities where such a license is required – are perfectly kosher.

On a side note, going by the fantastic work done by fictional Private Investitgator characters like Dennis “Cisco” Wojciechowski in Michael Connelly novels and Kalinda Sharma in the TV show The Good Wife, I think PI is a very under-appreciated profession.

UPDATE DATED 2 NOVEMBER 2021:

(added this in response to comments)

I’ve used the term “state” in my original answer as a proxy for “government” at federal, state and city levels.

FBI and Police do farm out investigation work to Private Investigators. And it’s not just when the state falls short of manpower, as I’d mentioned in my original answer.

Per Private Investigator Law Enforcement vs. Private Investigator, there are more situations under which the outsourcing happens:

Private investigators often assist in criminal investigations and can even be found working alongside law enforcement in certain instances.

Under the law, a police officer has restrictions on his or her actions that a PI may not be bound by. For this reason, police often hire private investigators when they can’t pursue the case further.

In addition, the law can only step in once sufficient evidence has been obtained, evidence that a PI may be able to track down. Thus, a private investigator may be hired to perform a criminal investigation only until enough evidence has been collected so that police can take over and make an arrest.

Thanks to budget cuts across the country, sometimes private investigators lend assistance not in addition to, but in place of the police when they don’t have the manpower available.

UPDATE DATED 11 NOVEMBER 2021:

Here are some specific areas in which FBI and Police Departments work with Private Investigators.

1.

FBI in Cyber Crime.

Business and Industry Partners | Federal Bureau of Investigation
Partnering with the FBI can help your company better understand the current risk environment and enhance our response in the event of a cyber attack.
https://www.fbi.gov/investigate/cyber/partnerships

2.

Police Department in Intellectual Property: “… many of these investigations are successfully pursued by law enforcement officials.” They also provide “training to numerous law enforcement agencies” ~ Home – Investigative Consultants.

3.

A leading police department acknowledges the contribution of Private Investigators in the busting of a counterfeit ring.

Counterfeit Ring Busted – LAPD Online
Los Angeles: On August 18, 2005, at about 4:00 p.m., Central Area Vice in conjunction with private investigators from Investigative Consultants served a search warrant for counterfeit merchandise at a business located in the 1100 block of South Los Angeles Street.  Investigative Consultants conducted surveillance of the business for about one month before they contacted … Continue reading “Counterfeit Ring Busted”
https://www.lapdonline.org/newsroom/counterfeit-ring-busted/
NOTE: I don’t remember the name but there’s this Michael Connelly novel in which Detective Harry Bosch is serving his suspension from LAPD. He uses the downtime to carry out some investigation for his stepbrother, Defence Attorney, Mikey Haller. There’s no end to the ribbing he gets from his cop coworkers for “crossing over to the other side”. Although that was a work of action, it’s clear from some of the comments to this answer that there’s no love lost between cops and PIs even in real life!

 

*: This is the original question I answered. I’m repeating it to help me make sense of my answer in case it’s moved to / merged with some other question that I didn’t answer.