In the 1961 case of Monroe v. Pape, the Monroe family filed suit against the Chicago PD and several individual officers for an illegal search of their home and an unlawful detention of Mr. Monroe. And they won. But the city of Chicago appealed all the way to the Supreme Court arguing that the Monroes could only sue the officers and not the city. The Supreme Court ruled in favor of the city.
Still, Monroe was a very important case. It marked the first time that a major police department had been sued for a federal civil rights violation and the plaintiffs prevailed. Some 20 odd years later, the Supreme Court changed its mind in the Monell case. The details of Monell don't really involve law enforcement. However, the decision in Monell made government entities vulnerable to civil rights lawsuits that stem from the actions of their employees.
The plaintiffs in both Monell and Monroe contended that their civil rights had been violated and filed suit under a Reconstruction-era law (The Ku Klux Act of 1871) that was still on the books as Article 42, Section 1983. In a nutshell, this law says that government employees are civilly liable if they deny a person his or her civil rights while on duty. It was a good thing.
But Congress decided it wasn't enough of a good thing, so in 1976, it passed the Civil Rights Attorney Fee Award Act. This law was added to the U.S. Code as Title 42, Section 1988. And what it does is basically make it really profitable for attorneys to sue government entities for civil rights violations.
Attorneys representing clients in basic tort cases usually score about 35 to 40 percent of the award if their client wins a judgment. In Section 1983 civil rights cases not only do prevailing plaintiff attorneys collect a percentage of their clients' award, they can also submit a bill to the court and, basically, the defendant has to pay it. In some cases, these attorney's fees have exponentially exceeded the size of damages awarded to the plaintiff.
Section 1988 is why the overwhelming majority of lawsuits filed against police officers and their agencies include both state and federal causes of action. It codifies windfall profits for personal injury and criminal lawyers who dabble in suing cops.