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Weeks before the start of the trial of an Iowa man accused of rigging lottery jackpots for years and pocketing millions in winnings, a U.S. Senate oversight panel is demanding to know what lottery officials are doing to prevent it from happening again.

In a letter to the Multi-State Lottery Association (MUSL) officials released on Wednesday, U.S. Senator John Thune (R – South Dakota), chairman of the Committee on Commerce, Science and Transportation, wrote, “Other than seeking new leadership, it is unclear what additional measures MUSL is implementing to provide effective oversight to ensure the integrity of lottery games.”

Thune, whose committee has subpoena power, wants to examine the results of an internal MUSL investigation that led the organization to its own security director, Eddie Ray Tipton — who was convicted last year on two counts of felony fraud for rigging a $16.5 million Hot Lotto jackpot in Iowa.

Thune also wants to know what steps the non-profit cooperative of 37 state lotteries has taken since to ensure the integrity of lottery drawings for so-called ‘pick six’ games like Hot Lotto going forward.

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After Tipton’s April, 2015 arrest, MUSL officials said they fired him and replaced the computers he worked on.

Tommy Tipton (above) turned himself in to face charges that he conspired with his brother, former lottery vendor Eddie Tipton, to win rigged jackpots worth millions of dollars in Oklahoma and Colorado. Polk County Jail via AP file

"The Multi-State Lottery Association welcomes the interest of the Committee and will thoroughly and promptly respond to the questions raised by Senator Thune," a MUSL spokesman said in a statement released Thursday.

Tipton is facing a second trial in July for allegedly rigging lottery games in four other states to produce jackpots worth more than $2 million. Tipton, who declined interview requests from NBC News, has maintained his innocence and is appealing last year’s verdict, according to his attorney.

The $16.5 million jackpot was never disbursed because officials became suspicious of efforts to claim the prize and launched an extensive investigation that led them to Tipton, who was captured on surveillance video purchasing the winning ticket at an Iowa convenience store. Under state law, lottery employees are barred from playing the games.

The association also suspended longtime director Charles Strutt, who has since retired, but has yet to name a successor. Beyond that, the association, a non-profit cooperative that serves 37 states and U.S. territories, has not released any further information about the results of its internal probe.

Prosecutors from the Iowa Attorney General’s office say that Tipton installed an undetectable computer code into random number generator computers in at least five states that pre-selected a set of numbers which either he or his associates then played and won. All of the allegedly rigged drawings occurred between 2005 and 2011 on either Nov. 23 or Dec. 29.

In all, prosecutors say they have tied Tipton to rigged lottery games in Iowa, Colorado, Oklahoma, Wisconsin and Kansas. The investigation is ongoing.

Tipton’s brother, Tommy and a business associate, Robert Rhodes, have also been charged and are awaiting trial.

Thune has given MUSL until July 15 to brief his committee. Tipton's second trial is scheduled is expected to begin later this summer.

MUSL also generates winning numbers for Powerball, but those jackpot combination are picked through manual “ball drop” drawings in which numbered balls are chosen randomly from clear containers.