Prosecutorial immunity means that you cannot sue the District Attorney except in limited circumstances. The police can also invoke that immunity if they acted at the DA's direction, but that does not help the police in this case.
The case is Simon v. City of New York, decided on August 16. The general rule is that disgruntled people cannot sue the DA, lest the DA be sued each time he prosecutes someone. But if the DA is doing something in his administrative capacity -- as opposed to something he did in his prosecutorial function -- he only gets qualified immunity, which can be overcome if the facts show a clear civil rights violation. In this case, the DA's office mistakenly thought that Simon was a material witness in a car theft case (they confused Simon with her daughter, who had a similar name). Simon was taken against her will to the precinct on a warrant. She told the police she knew nothing about a stolen car. The defendants, including the police, tried to get the case against them dismissed on prosecutorial immunity, but the Court of Appeals reverses summary judgment and remands the case.
As the Second Circuit (Walker, Katzmann and Lynch) reminds us, "a prosecutor acting in the role of an advocate in connection with a judicial proceeding is entitled to absolute immunity for all acts intimately associated with the judicial phase of the criminal process." This includes deciding whether to bring charges, witness preparation and issuing subpoenas. They only get qualified immunity for "administrative duties and investigatory functions that do not relate to an advocate's preparation for the initiation of a prosecution for judicial proceedings." This includes investigation, arrest and detention, as well as giving legal advice to the police in the investigative phase of a criminal case, or assisting in a search and seizure or an arrest.
Even though they were following the ADA's orders, the police in this case do not have absolute immunity for plaintiff's detention against her will. Executing a material witness warrant is a police function, not a prosecutorial function. Since the police acted under the ADA's direction in executing the warrant but failed to adhere to the warrant's direction to bring plaintiff to court (they took her to the police station instead), they cannot invoke absolute immunity. This was an investigative interview, not the kind of witness preparation for which absolute immunity attaches.
Although the Second Circuit's decision is not clear on this, this holding likely applies to the ADA as well as the police officers. The ADA was a defendant-appellee and the district court threw out the case against all defendants. That ruling is reversed in its entirety. So here's the rule: detaining someone pursuant to a material witness warrant is not a prosecutorial function entitling law enforcement to prosecutorial immunity.