BARCELONA, Spain (AP) — Spanish police arrested a former aide to Venezuela's ambassador to the United Nations on a U.S. warrant in connection with a $1 billion bribery scheme at Venezuela's state-run oil company, officials said Friday.

Spain's Civil Guard said Rafael Reiter was detained in Madrid as part of a months-long sting ordered by the U.S. Department of Homeland Security. On Thursday, three other former Venezuelan officials were arrested on money laundering charges brought by prosecutors in Houston, including a former deputy energy minister also close to Rafael Ramirez, who for a decade was Venezuela's oil czar and since 2014 its ambassador to the U.N.

The four men are the most-senior former officials detained as part of an ongoing U.S. probe into corruption at state oil company PDVSA. Their arrest suggests that after securing guilty pleas from 10 individuals for their role in the payment of bribes and kickbacks, prosecutors are turning their attention to the Venezuelan officials that pocketed huge sums in exchange for the fraudulent contracts. In 2015, the U.S. Treasury Department accused a bank in Andorra of laundering some $2 billion stolen from PDVSA.

Reiter, 38, was the head of PDVSA's corporate security unit who sometimes appeared alongside Ramirez in public.

Alejandro Antonini Wilson, a star witness in an unrelated U.S. case against five Venezuelan government agents scheming to smuggle $800,000 into Argentina, said that Reiter was a very close, personal assistant to Ramirez. In the 2008 case he and others testified that Reiter was the person who loaded onto a chartered plane flying from Caracas to Buenos Aries two suitcases containing the cash that prosecutors said was destined to finance the election campaign of former President Cristina Fernandez. Several PDVSA officials were on the plane.

"There are lots of people that are not sleeping well tonight with these arrests," Wilson told The Associated Press on Friday.

Ramirez has not been charged, but has previously dismissed the U.S. probe into PDVSA as a politically-motivated attempt to undermine President Nicolas Maduro's socialist government. Repeated calls to his office at the U.N. on Friday went unanswered.

Much of the evidence gathered by prosecutors stems from a case in Houston against two Venezuelan businessmen residing in the U.S., Roberto Rincon and Abraham Shiera, who in 2015 were charged with violating the foreign corrupt practices act by conspiring to pay bribes in exchange for contracts to build power generators for PDVSA at a time Venezuela was suffering from widespread outages.

U.S. prosecutors in their indictment accused the two men of paying everything from a PDVSA official's $165,000 mortgage in Texas to picking up a $15,000 reservation for several company officials at the Fontainebleau Hotel in Miami Beach in order to win business.

Both men are out on bail awaiting sentencing. Prosecutors in Houston and the U.S. Justice Department declined to comment.

Also taken into custody in Spain was Nervis Villalobos, who for years was deputy energy minister in charge of electricity projects and on several occasions stepped in for Ramirez when his boss was on missions abroad. He was arrested along with Luis Carlos de Leon, a former official at a state-run electric company in Caracas, and Cesar David Rincon, a former senior executive at PDVSA's procurement unit, Bariven, who later set up a consulting business.

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AP Writer Jorge Rueda contributed to this report from Caracas, Venezuela and Claudia Torrens from New York. Goodman reported from Bogota, Colombia