As the United States looks to bolster ties among anti-ISIS allies, it should heed the call of the Turkish government to extradite Fethullah Gülen, the leader of what some describe as an Islamist cult run out of a million-dollar estate in the Poconos.

All Americans realize that one of the biggest threats facing our country comes from the Islamic State, the same terrorist hydra that threatens to mirror the Paris attacks on our homeland. Off to a slow start, the White House-led battle to defeat ISIS will be a long one which will need a broad coalition of nations to succeed.

Arguably, our most important ally in the fight is Turkey, our NATO partner, which borders some of the territory under ISIS control. There has never been a moment when we needed stronger, more enduring ties with a partner facing a mutual threat. Yet Washington is neglecting this key ally. We are rebuffing a direct Turkish request: the extradition from the U.S. of a fugitive Turkish imam who faces charges of terrorism back home. Even a personal plea from Turkish President Recep Tayyip Erdoğan to President Barack Obama has been ignored since the two met during the NATO Summit in 2014.

Fethullah Gülen, a 74-year-old billionaire cleric, has lived in a multi-million-dollar estate in the Poconos since 1999, when he took up residence in this country in self-imposed exile. His reach is broad and deep both overseas and here in the U.S., mostly under the cloak of an organization known as the Gülen Movement. Supporters paint it as the benign, modern, moderate face of Islam. Critics point to a contrasting, more Islamist bent, as shown in a classified State Department cable from Ankara as revealed by Wikileaks.

Last fall, after Gülen was implicated in a string of criminal inquiries for attempting to overthrow the democratically elected Turkish government, a Turkish court issued a warrant for his arrest. With a legitimate arrest warrant out for him and a proper extradition request in place, one might think our country would be supporting the NATO ally Turkey and bolster the relationship by honoring the warrant. But we’re not.

Clearly, the U.S. and Turkey should stand as one in the fight against ISIS. As Turkey is considering sending its own forces in to battle ISIS, we agree on the need to oust Assad and to lessen Russia’s influence in the region. But the White House doesn’t stand together in the matter of Gülen, who is fighting Turkish extradition requests.

And there is more: Turkey isn’t the only place he is under the scrutiny of the law. Here in the U.S., his organization is directly or indirectly the subject of multiple ongoing federal and state investigations.

The Gülen Movement controls over 140 taxpayer-funded charter schools spread out over 25 states. Some claim the schools are used to proselytize Gülen’s version of Islam to American students. The organization is also accused of using the schools as a pretext to bring thousands of young Turkish men into America as teachers under H-1B visas, which are traditionally used to employ foreign workers in specialty occupations that require technical expertise and for which there aren’t enough U.S. workers already. Turkish teachers of English are not in that category, yet they are being imported in droves, a flagrant abuse of our immigration system.

Separately, a USA TODAY investigation concluded that the Gülen Movement may have directed hundreds of thousands of dollars in questionable campaign donations to congressional and presidential candidates during the past several years. The investigation identified dozens of large campaign donations attributed to people with ties to the Gülen Movement, modest incomes, and having little to no knowledge of the candidates to whom they had given.

But Gülen hasn’t stopped there. Reports also indicate that a dozen different entities tied to the Gülen Movement have secretly and in some cases illegally funded more than 200 foreign trips for members of Congress and their staff, repeatedly violating House rules and perhaps federal law.

Additionally, Gülen and close associates have reportedly donated hundreds of thousands of dollars to Hillary Clinton’s campaign and over one million dollars to the Clinton Foundation. Some political figures who’ve received Gülen-linked monies returned them upon learning more about the source. Not so with Hillary.

Gülen’s motives in using his network of organizations to influence students, elections and politicians here in the U.S. are unclear, although a 1999 video shows Gülen telling his followers that they should deliberately attempt to infiltrate mainstream structures, “moving without anyone noticing your existence until you reach all the power centres.”

Where there’s smoke, there’s fire: in this case red flags clearly implying a need for a comprehensive investigation. For our part, the White House must support Turkey’s longstanding role as an ally and a friend. Deporting Fethullah Gülen back to Ankara would be the right thing to do. So would Hillary’s returning the contributions she received from Gülen and his associates.

Bill Cowan is a retired Marine Corps Lieutenant Colonel who appears regularly on the Fox News Channel and the Fox Business Network.