A Nigerian man who masterminded a conspiracy to obtain millions in refunds from the Internal Revenue Service has been sentenced in Oregon to 15 years in federal prison.Emmanuel Kazeem, 35, who also lived in Bowie, Maryland, was sentenced Wednesday in Eugene.A jury previously convicted him of fraud, conspiracy and aggravated identity theft."Emmanuel Kazeem orchestrated one of the largest tax fraud schemes in our nation's history. The complexity of this case and the incredible effort by law enforcement to bring those responsible to justice cannot be understated," U.S. Attorney for Oregon Billy Williams said Thursday.An IRS criminal investigation found that Kazeem purchased more than 91,000 identities from a Vietnamese hacker that originated from an Oregon company's private database. They were used to file fraudulent tax returns between 2012 and 2015.Investigators determined that Kazeem was linked to more than 10,000 fraudulent federal tax returns in an attempt to score $91 million in refunds. Conspirators received $11.6 million, using pre-paid debit cards with the victims' stolen identities to receive direct electronic deposits.Prosecutors said Kazeem used the ill-gotten gains to buy a house in Maryland and put a nearly $200,000 down payment on another. He also tried to use the money to develop a four-star hotel in Lagos, Nigeria. To illustrate his lavish spending, prosecutors said Kazeem's average monthly credit card payment topped $8,300 during the scheme.The authorities began their investigation after an Oregon woman alerted the IRS in May 2013 that false federal and Oregon state tax returns were filed electronically using her and her husband's names. The returns included personally identifiable information, including their Social Security numbers and dates of birth.Kazeem was arrested in May 2015, one day after he transferred a Maryland home to his sister in Nigeria for $10.The U.S. Attorney's office said Kazeem came to the United States on a student visa and entered a fraudulent marriage to remain in the country. He will be deported after his release from prison.Kazeem's brother, Michael, and four other co-conspirators previously received prison sentences in the case.