HARTFORD, Conn. - Connecticut Attorney General Richard Blumenthal says his worries over a drug company's products have been substantiated, and his investigation will continue.

Cephalon told doctors Monday that inappropriate prescribing of its cancer pain drug Fentora has resulted in three deaths, most likely due to respiratory failure. Blumenthal last year initiated an investigation into the company allegedly promoting some drugs for purposes they weren't intended.

"Today's report by Cephalon tragically confirms concerns raised by our aggressive investigation into Cephalon's off-label marketing of its drugs," Blumenthal said. "News of confirmed deaths and other adverse reactions adds urgency and immediacy to our active and ongoing investigation."

Cephalon has been subpoenaed by Congress to testify about off-label prescriptions, when doctors prescribe a drug for a purpose other than its intended. The company is trying to gain approval for Cephalon to be used to treat back pain, though two of the deaths occurred in consumers using the drug for a migraine and the other was the result of improper dosage, a report by The Associated Press said.

"Our serious and significant concerns about this drug, already substantiated by evidence, are likely to support efforts for remedial action," Blumenthal said.

"Although doctors are permitted to prescribe drugs for off-label uses, drug companies are prohibited from marketing off-label uses, which can encourage improper and potentially dangerous prescribing and lead to increased drug diversion.

"Our investigation is continuing vigorously, and we expect to announce the results of our findings in the near future."