A U.S. federal judge in New York sentenced Bout to 25 years in prison Thursday for selling weapons he intended to be used to kill U.S. citizens. The so-called merchant of death was convicted in November on charges of arms trafficking and terrorism conspiracy.
Global Witness said Bout was able to hide behind shell companies set up in the United States and overseas.
"He exploited the same loophole used by terrorists, corrupt dictators, drug traffickers and tax evaders to legally hide their identities to access the U.S. financial system," Stefanie Ostfeld, a policy adviser for Global Witness, said in a statement.
The advocacy group points to a World Bank report that found the United States was a "favorite destination" for corrupt officials because of a lack of financial transparency laws.
"Once corrupt and other dirty money has been moved through an anonymous corporate vehicle into the financial system, it is much harder to track it down," said Ostfeld.
Bout, a former Soviet military officer, was arrested in Thailand in March 2008 in a U.S. Drug Enforcement Administration sting that used informants posing as representatives of a Colombian terrorist group, the Revolutionary Armed Forces of Colombia.
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