Updated

A Texas doctor wrote unnecessary prescriptions for powerful drugs that contributed to the overdose deaths of at least seven people over a four-year period, according to a federal indictment unsealed Wednesday.

Howard Gregg Diamond, 56, was arrested Tuesday on charges that include conspiracy to distribute controlled substances and health care fraud.

Prosecutors contend that Diamond began issuing prescriptions in 2010 that had no legitimate medical purpose. The prescriptions were for drugs such as fentanyl, hydrocodone and morphine, according to the indictment.

Authorities say the overdose deaths occurred in the Texas cities of Abilene, McKinney and Sulphur Springs, and in the Oklahoma cities of Ardmore, Hugo, Idabel and Yukon.

In each case, the seven people died within a month of filling the prescription provided by Diamond, the indictment states. One person filled a prescription for methadone in May 2013 and was dead two days later.

In another instance, a woman only identified in the indictment by the initials T.H. filled a prescription in July 2014 for alprazolam, morphine, oxycodone and zolpidem. She died 10 days after receiving the drugs.

Diamond, who is based in Sherman, about 60 miles (97 kilometers) north of Dallas, pleaded not guilty to the charges on Tuesday. He is due in court again on Friday for a detention hearing. He is in federal custody and could not be reached for comment. His public defender, Denise Benson, didn't immediately reply to a message Wednesday seeking comment.

The indictment alleges that Diamond conspired with others to distribute the drugs but there was no indication whether additional arrests will be made.

Brit Featherston, the acting U.S. attorney for the eastern district of Texas, declined Wednesday to discuss the ongoing investigation but said the opioid problem has been getting worse. He said area law enforcement officers are seeing more and more cases of parents driving under the influence of opioids with their children in the back seat, indicating the prescription drugs are no longer confined to back alley deals.

"It's a different type of problem that we've been seeing build for several years now," he said.

Diamond also is accused of defrauding Medicare of tens of thousands of dollars by filing false claims and through other means.

Prosecutors say they'll seek to have Diamond's medical license revoked.

Online records kept by the Texas Medical Board show that Diamond's medical license was issued in 1988. He appears to have specialized in spinal injuries and rehabilitation.

The board in 2015 required Diamond to submit to a remedial plan after finding he "failed to maintain adequate medical records documenting his care and treatment of chronic pain patients." Records indicate the plan concluded when he completed its requirements a year later.

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An earlier version of this story was corrected to show that Diamond was arrested Tuesday, not Monday.