A former Casper doctor accused of running a multistate opioid conspiracy allegedly threatened to shoot a customer in his office, according to a federal court document filed Thursday.

An eyewitness told prosecutors she saw a "'furious'" Shakeel Kahn confront the customer in the lobby, and accused the customer of trying to get opiates from other doctors, according to the document.

Kahn went to the back of his office and returned to the lobby with a handgun.

"S. Kahn pointed the handgun at the customer's head and threatened to kill the customer," according to the prosecutor. "The witness yelled at S. Kahn and S. Kahn eventually lowered his firearm and returned to the rear of the office."

The allegation does not state whether the incident occurred at Kahn's former Casper office on South Fenway Street or in Arizona, where he had another office.

The allegation was contained in a notice to introduce 404(b) evidence -- evidence of prior bad acts of a defendant -- at Kahn's drug conspiracy trial scheduled in Casper starting April 29.

The request by the prosecution to introduce 404(b) evidence must be decided by the judge because of guidelines that require a proof of motive, opportunity and other factors that are relevant to the case or are not unduly prejudicial, according to the document. "The evidence is relevant to the allegations in the indictment that S. Kahn prescribed opioids and other medications outside the usual course of professional practice and without a legitimate medical need."

The federal government charges some or all defendants -- Shakeel Kahn, his wife Lyn (Voss) Kahn, his brother Nabeel "Sonny" Kahn, and Arizona resident Shawwna Thacker -- with conspiracy to distribute, possess and dispense oxycodone and other prescription opioids; conspiracy to distribute certain prescription drugs resulting in death; possession or use of firearms during a drug crime; unlawful use of communications devices; and money laundering.

Shakeel and Lyn Kahn dealt with customers who paid them $500 in cash for a prescription, according to court records. Medical evaluations were usually minimal if not nonexistent. Customers often would resell the drugs. Some of the transactions occurred at the now-closed Vape World shop on 12th Street operated by Kahn's stepchildren.

Shakeel Kahn also is charged with operating a continuing criminal enterprise, which upon conviction is punishable by a minimum of 20 years of imprisonment.

The charges include using firearms in the furtherance of the conspiracy.

Kahn threatened the customer as an attempt to intimidate the customer from contacting a legitimate physician for pain medication, which in turn could cut into Kahn's profits or draw negative attention to him, according to the document.

"This display of force could also serve to intimidate others who might have considered seeking paid medication from other sources of supply. The evidence also demonstrates that S. Kahn was willing to use firearms and threats of violence to ensure the success of his drug distribution enterprise."

Besides the allegation of threatening to shoot the customer, the document cites other evidence of prior bad behavior:

-- While a resident at the University of Wyoming Family Medicine Residency Program in Casper, Kahn was evaluated in 2007 by two physicians who were concerned about Kahn's prescribing opioids.

-- The Arizona Medical Board in 2016 reviewed Kahn's patient charts, and identified deviations from the standard of care regarding prescribing opioids. The Board notified Kahn that it would suspend his license; Kahn appealed the suspension; and the U.S. Drug Enforcement Administration notified him that his DEA registration was subject to revocation.

-- As part of the conspiracy, Nabeel and Shakeel Kahn committed insurance fraud in 2015 when Nabeel Kahn used Shakeel Kahn's insurance to pay for a partial amputation of his foot. The subtotal of charges was $314,972.14. Shakeel Kahn also prescribed Lyn Kahn's daughter with multiple prescriptions of oxycodone and hydrocodone for pain including "'foot and ankle pain,'" and the daughter diverted the drugs to Nabeel Kahn.