He was charged with disorderly conduct and resisting arrest, but prosecutors dropped the charges, saying there was “no reasonable likelihood of successful prosecution.”

Mr. Lozman sued, saying he had been arrested in retaliation for exercising his First Amendment rights. The arrest was payback, he said, for his criticism of the city’s plan to redevelop the waterfront by taking private property using eminent domain.

The federal appeals court in Atlanta said Mr. Lozman had made a compelling point. “He seems to have established a sufficient causal nexus between Councilperson Wade and the alleged constitutional injury of his arrest,” the court said in an unsigned opinion in February.

Despite that, the court ruled that Mr. Lozman could not sue. Since there had been probable cause for the arrest, the court said, it did not matter that it might also have been motivated by retaliation.

Oddly, the court did not say there had been probable cause to arrest Mr. Lozman for the crimes he had been charged with. Instead, the court said the officer had had probable cause to believe that Mr. Lozman “was committing, or about to commit, the offense of disturbing a lawful assembly.”

In an interview, Mr. Lozman said he found this perplexing. “Can you arrest somebody and come up with some probable cause argument even if it’s eight years later?” he asked.

The question of whether the existence of probable cause is always enough to defeat a lawsuit claiming retaliatory arrest has divided the appeals courts. The Supreme Court agreed in 2011 to decide the question but ended up ducking it.