Topped with the word “subpoena” and the seal of the district attorney in New Orleans, the documents carried an air of authority. They instructed people to appear before prosecutors “to testify to the truth,” and they warned that “failure to obey” the missive could lead to a fine and imprisonment.

But the personalized documents were never endorsed by any court. Instead, according to a federal civil rights lawsuit brought on Tuesday in New Orleans, the papers that were disguised as subpoenas were central to a sustained and fraudulent effort by local prosecutors to coerce witnesses.

The lawsuit is an escalation of a controversy that emerged this year in Louisiana’s most populous city, but it also brings new scrutiny to the bare-knuckled tactics that officials across the country sometimes use to build cases.

The lawsuit, filed by seven plaintiffs, accused local prosecutors of menacing prospective witnesses with what were supposedly subpoenas. Sometimes, the lawsuit said, officials asked judges that people be jailed as material witnesses after they balked at the demands for private meetings with prosecutors.