Turkish President Recep Tayyip Erdogan in Ankara on Friday. (Adem Altan/AFP/Getty Images)

THE DAY before then-Secretary of State Rex Tillerson met with Turkish President Recep Tayyip Erdogan in February, federal prosecutors dropped criminal charges against seven members of Mr. Erdogan’s security detail charged in last year’s attack on peaceful protesters outside the Turkish ambassador’s residence in Washington. Mr. Erdogan had long made clear he was furious about the cases, and they were one factor in tensions between the two countries. But officials said that the timing of the dismissals was purely coincidental and that there was no political pressure.

Let’s hope that’s true. Mr. Erdogan’s thugs deserve to be brought to justice, whether their arrogant leader likes it or not. If their cases were to be dropped for diplomatic reasons, the Trump administration at least should have demanded concessions in return — starting with the immediate release of Christian missionary Andrew Brunson and other Americans suffering in Turkish jails. Mr. Erdogan himself has described the Americans he is holding as bargaining chips; he would like to use them to obtain the arrest and extradition of a Turkish political enemy living in Pennsylvania.

Even if the bodyguards’ cases were dismissed on the merits, there are troubling questions about the government’s handling of the unprovoked attack on people protesting Mr. Erdogan’s regime during his visit here last May. Videos of the incident outside the ambassador’s residence show men in suits, some with guns, kicking and punching protesters as police try to intervene. Nineteen people, including 15 members of Mr. Erdogan’s security team, were indicted. Charges against four Turkish guards were dropped in November; on Feb. 14, the day before Mr. Tillerson’s trip to Ankara, charges against seven more guards were dropped. Charges remain against four guards, although the possibility that they will be arrested and brought to trial is remote.

“We don’t comment on charging decisions and have no comment on this case,” the spokesman for the U.S. attorney said in a statement when the dismissals became public, which happened only after a query from the Wall Street Journal. The Journal cited problems with the investigation, including issues with identification and a lack of sufficient evidence. That police and prosecutors, who reportedly had the assistance of the State Department in identifying people, could fall so short in probing such a high-profile case — one touching on American principles of free assembly and dissent — doesn’t inspire confidence.

Still, if the U.S. justice system rigorously and fairly evaluated the cases against Mr. Erdogan’s musclemen and determined they could not all be fairly prosecuted, then perhaps that could serve as a useful model for the Turkish leader. A number of innocent U.S. citizens, and many thousands of innocent Turkish journalists and other political prisoners, would be the beneficiaries.