LITTLE ROCK — The Arkansas Medical Board on Thursday voted to end its investigation into how drugs used to execute four inmates in April were obtained after the panel's attorney said he found no evidence that any doctors licensed by the state were involved.

Last month, the board had said it was investigating the purchase of one of the drugs used in the state's first executions in nearly 12 years. But board attorney Kevin O'Dwyer said Thursday that he didn't find any evidence of a violation of the state's medical practices act by a licensed doctor because none was involved in obtaining the execution drug.

"There was no doctor that we found that was involved in procuring the drugs," O'Dwyer told reporters after presenting his findings to the board. The panel voted to take the case "for information," meaning it would no longer pursue the investigation absent any new information.

Doctors can participate in executions under Arkansas law. But questions were raised in a court case about whether Arkansas improperly used a doctor's name and license to purchase one of the drugs. The Department of Correction has denied those claims. O'Dwyer said the investigation focused on whether a doctor was involved in obtaining the drugs or allowed their license to be used to obtain them.

"The very narrow investigation we would have was whether our licensee was involved, and he doesn't appear to be," O'Dwyer said.

O'Dwyer said the investigation focused on the state's purchase of vecuronium bromide, one of three drugs used in its lethal injection protocol. McKesson Corp. asked a judge to block Arkansas from using its supply of vecuronium bromide, claiming it had not intended the drug to be used for executions. The Arkansas Supreme Court in April stayed a judge's order preventing the state from using the drug, and a lawsuit over the company's claims is pending in Pulaski County court.

Arkansas had originally intended to put eight inmates to death over an 11-day period in April, but courts halted half of the executions. The executions had been scheduled to take place before the state's supply of midazolam, a sedative used in its lethal injection protocol, expired at the end of April. Arkansas has not obtained a new supply of the drug.

Autopsies showed that each of the four inmates had the sedative midazolam in their systems, a newspaper reported Thursday, but tests were not performed for two other execution drugs intended to paralyze the inmates and stop their hearts. The Arkansas Democrat-Gazette reported that lab work did not include searches for vecuronium bromide or potassium chloride.

A federal public defender, Scott Braden, told The Associated Press the results make it difficult to determine whether the state followed its own protocols. "We don't know what went in when," Braden said.

Prison officials said the drugs were administered in the proper order, with the sedative given first. The state's medical examiner, Dr. Charles Kokes, did not return telephone calls seeking comment.

A federal judge, acting on a request from the inmates' lawyers, has ordered the state to retain material for possible further testing.