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The senior ISIS leader killed in an overnight raid in Syria held a central financial role in the terrorism organization, but little else is known about Abu Sayyaf.

Abu Sayyaf, who was killed by U.S. Special Operations Forces, helped direct ISIS's illicit oil, gas and financial operations, which are "a key source of revenue that enables the terrorist organization to carry out their brutal tactics and oppress thousands of innocent civilians," the White House National Security Council spokeswoman Bernadette Meehan said in a statement.

He was a Tunisian citizen whose wife, Umm Sayyaf, is also considered to be an ISIS operative. Special Operations Forces took Umm into custody after killing Abu Sayyaf and 12 enemy fighters at his compound.

Abu Sayyaf was known to be close to ISIS leader Abu Bakr al-Baghdadi. A U.S. military official called Abu Sayyaf the "emir" of the group's prosperous oil and gas transactions.

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But little else is known about him.

"In short, we've never heard of Abu Sayyaf before, or his wife," said Evan Kohlmann of Flashpoint Intelligence, an NBC News analyst. "There are a variety of reasons that could account for this, not the least of which is that ISIS obviously doesn't have much to gain by publicizing the identity of the person in charge of their underground oil trade."

Sayyaf is not known to be among the terrorists that the U.S. has offered a bounty for, and numerous Middle East experts were unable to offer any additional information on him.

A terrorism analyst who was not identified told the New York Times that Sayyaf was easily replaceable, comparing him to Al Capone's accountant.

The U.S. was hoping to capture him alive during the pre-planned mission so that he could be questioned. Officials said his wife, Umm Sayyaf, who is being held in Iraq away from other prisoners, is speaking to interrogators. A U.S. official said she is accused of being "directly complicit in (ISIS) activities, specifically human trafficking."

"We are also working to determine any information she may have regarding hostages — including American citizens who were held by (ISIS)," Meehan said in a statement.

In the end, it may not be Sayyaf and his wife that were the big score. According to U.S. military officials, "his communications equipment and belongings are a valuable source of intelligence in learning how ISIL finances operations and senior leaders communicate."

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