An organization of private citizens that cooperated with the Justice Department and the Bureau of Investigation during World War I to spy on German residents suspected of disloyal behavior.

That organization was the American Protective League.

Explanation:

It had a network of spies or collaborators in 600 cities. It used a variety of methods such as harassment, intimidation, surveillance and outright violence. The League was the largest among several organizations working for those purposes. Unlike other organizations, its actions had the approval of the US official agencies.