He Disparaged the Police on Facebook. So They Arrested Him.

The defendant in the case is Gordon MacDonald, New Hampshire’s attorney general. A spokeswoman for his office declined to comment, but a memorandum on Mr. Frese’s case from a lawyer in his office appears to set out his basic position.

The lawyer, Elizabeth A. Lahey, said the state’s criminal libel law was constitutional, as it required prosecutors to prove that Mr. Frese knew he was saying something false in criticizing the police chief. (That is an even higher bar than the one required in civil libel cases brought by public officials. Under the Supreme Court’s 1964 decision in New York Times v. Sullivan, they must prove “actual malice” — that the defendant knew what he said was false or acted with reckless disregard as to the statement’s truth.)

The problem with the prosecution of Mr. Frese, Ms. Lahey wrote, was that the authorities had no reason to think he believed his statements were false. The Exeter Police Department dropped the prosecution after getting Ms. Lahey’s memo.

Letting government officials prosecute their critics for supposed misstatements is a dangerous business. But not every criminal libel prosecution concerns official conduct, and people who have studied the matter say criminal libel prosecutions may have a role to play in vindicating damaged reputations.

Professor Volokh, an authority on the First Amendment, said narrowly written criminal libel laws were constitutional. A 1964 Supreme Court decision, Garrison v. Louisiana, struck down a state criminal libel law, but only because it did not require proof of actual malice in cases concerning public officials. The majority opinion, from Justice William J. Brennan Jr., suggested that criminal laws that included an actual malice requirement were permissible.

Mr. Hauss, the A.C.L.U. lawyer, said the Garrison decision did not address the argument that criminal libel laws are unconstitutionally vague.

I asked Professor Volokh whether criminal libel laws were good policy.

“That’s hard to tell,” he said. “On one hand, they can certainly be abused for political purposes, and they can deter even true statements, if the speaker is worried that a prosecutor and a jury will think the statements are false.”