For an overview on who Abdul Tawala Ibn Ali Alishtari (aka Abdul Tawala Ibn Ali ALLSHTARI and Michael Mixon) is here is a starting point. The short version is that he is a lifetime member of the National Republican Senate Committee's Inner Circle who has given $35k to various Republican organizations over the years. He is accused of donating over $100k to terror groups in Pakistan. He is currently being held on charges including terrorism financing, material support of terrorism and other charges in NY. He pleaded not guilty.

To date, the National Republican Senatorial Committee (NRSC) has donated his donations to a charitable organization. The National Republican Congressional Committee (NRCC) is still "extremely concerned and disturbed" but not enough to follow the NRSC and donate its portion to charity.

Reuters has the latest:

Spanish police have arrested a Canadian man [Brian David Anderson, 61] suspected of helping finance Islamist military operations, the interior ministry said on Monday.
...
It said Anderson was thought to be linked to Abdul Tawala Ibn Ali Alishtari, a New York businessman who has been indicted on terrorism charges in the United States and is also accused of funding Afghan military projects.

[UPDATE] The FBI was involved

FBI spokesman Richard Kolko said in Washington that his buro worked with international law enforcement and intelligence community partners to locate and arrest Anderson. He added that the investigation is continuing.