ATLANTA (Reuters) - U.S. authorities seized Georgia's supply of a drug used in executions on Tuesday because of concerns about how it was imported, a move praised by death penalty opponents.

U.S. Drug Enforcement Administration agents took control of the state's sodium thiopental, a sedative that attorneys for several death row inmates have said was improperly obtained.

"We commend the DEA for forcing the Department of Corrections to stop using black market execution drugs," said Sara Totonchi, executive director of the Southern Center for Human Rights in Atlanta.

Authorities offered few details about the motive for the seizure except to say there were questions about how the state had obtained the drug.

"DEA became aware of this situation today," Special Agent Chuvalo J. Truesdell said. "We took control of the controlled substances, and it's now a regulatory matter."

He declined further comment because of the ongoing investigation.

The Georgia Department of Corrections said it had requested federal assistance after questions were raised about how the drug was imported.

"We're working with them to make sure we're in compliance in the way we handle controlled substances," Corrections spokeswoman Peggy Chapman said.

The federal government stepped in following a letter sent last month to U.S. Attorney General Eric Holder on behalf of Georgia death row inmate Andrew Grant DeYoung.

With sodium thiopental in short supply nationally, Georgia corrections officials ordered the drug from a pharmaceutical distributor in London, England, DeYoung attorney John Bentivoglio wrote in the February 24 letter.

The state received 50 vials of sodium thiopental in July, Bentivoglio said, citing public records.

But Bentivoglio said the state was not registered to import the controlled substance and failed to notify DEA about the shipment.

"I think it raises very troubling questions about the lengths to which they would go to pursue lethal injections when that process requires careful attention to the integrity of the process," Bentivoglio told Reuters on Tuesday.

Totonchi said her center first raised questions about the state of Georgia's purchase from England in federal court filings for death row inmate Emmanuel Hammond, who was executed in January.

Sodium thiopental is the first of a sequence of three drugs administered in U.S. lethal injections that paralyze breathing and stop the heart. A sedative is legally required in all lethal injections of U.S. death row inmates.

Hospira Inc. of Illinois, the only U.S. company that manufactured sodium thiopental, said in January it would stop making the drug after Italy, where it planned to move production, objected because the European Union has banned the death penalty.

Several states have either run out of supplies of sodium thiopental or switched to using pentobarbital, a barbiturate that is often used to euthanize pets and other animals.

Georgia had 106 inmates on death row at the beginning of 2010, according to the Death Penalty Information Center. The state has no executions scheduled at this time, according to the Center, which tracks the death penalty in the states.

(Reporting and Writing by Colleen Jenkins; Additional Reporting by Matt Bigg; Editing by Greg McCune and Peter Bohan)