Behind the jubilation over the latest release of Americans unjustly imprisoned abroad are growing concerns that a new era of hostage diplomacy has emerged, with U.S. adversaries concluding that it pays to arrest innocent Americans and trade them for convicted smugglers, hackers, spies — and even killers.

While loath to criticize any specific swap, current and former U.S. officials worry that strongmen like President Vladimir Putin of Russia are exploiting America’s willingness to horse-trade for its citizens. Among those returned to Moscow on Thursday in a deal involving 24 prisoners and seven nations was a Russian assassin serving a life sentence in Germany for gunning down a Kremlin enemy in a Berlin park.

"I worry about the incentive this gives nations to abduct Americans,” said Adam Hickey, a former deputy assistant attorney general in the national security division of the U.S. Justice Department. Speaking of the trend toward hostage trades generally, he added: "I don’t see how this doesn’t lead to an escalation or increase” in the practice by foreign governments.