WASHINGTON (AP) — Turkey says the United States is legally bound by a treaty to immediately hand over Fethullah Gulen, the U.S.-based Muslim cleric it accuses of plotting to overthrow Turkey’s government.

The U.S. government says it can’t comply until Turkey can convince a judge its allegations against Gulen are legitimate.

Any solution lies in the murky world of extradition, where the U.S. criminal justice system overlaps with diplomacy and international law.

Unable to agree about the process, Turkey and the U.S. are feuding over Gulen, who denies involvement in the thwarted July 15 coup attempt. It’s become the biggest irritant between the two strategic partners just as they struggle to reconcile their approaches to fighting the Islamic State group across Turkey’s border in Syria.

During Vice President Joe Biden’s visit to Ankara this week, the disagreement played out in unusually sharp and open fashion. Both Turkey’s prime minister and president publicly badgered Biden and said the U.S. was harboring a terrorist, while Biden tried simultaneously to show sympathy and defend U.S. legal traditions.

“It’s never understood that the wheels of justice move deliberately and slowly,” Biden said.

A look at the case against Gulen and how extradition works:

Q: Why does Turkey want Gulen extradited?

A: Once an ally of President Recep Tayyip Erdogan, Gulen now lives in self-imposed exile in Pennsylvania. He’s associated with Sufism, or Islamic mysticism, and a founder of a movement known as Hizmet — “Service” in Turkish — that first expanded outside Turkey in the early 1990s after the Soviet Union fell. Gulen’s followers have established a network of schools around the world, and Turkey accuses Gulen of surreptitiously grooming students to eventually overthrow Turkey’s government. But U.S. officials say privately they’re skeptical about claims that Gulen was involved in the failed coup.

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Q. Has Turkey provided evidence that Gulen should be returned?

A: Yes and no. Turkey has submitted extradition requests for Gulen, but senior Obama administration officials say those requests were based on alleged crimes prior to the coup attempt. Turkey’s justice minister says more evidence relating specifically to the failed coup will be submitted next week.

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