ATLANTA (AP) — Some of the states scrambling to import a lethal injection sedative in scarce supply failed to properly register with federal regulators before receiving the drug from overseas, according to emails released this week.

The Drug Enforcement Administration was only formally notified twice last year that sodium thiopental had been imported, according to a November 2010 email sent to the agency, even though 10 states received the drug from overseas manufacturers, or another state that got it from outside the U.S.

The emails were obtained Wednesday by the ACLU of Northern California. Many of them emails were heavily redacted by the agency, and the DEA did not release any further details. It wasn't immediately clear which state notified the DEA about the imports.

States must register with the DEA before importing a controlled substance and notify the agency when they receive it. Defense attorneys in Georgia and elsewhere have said breaking those rules could mean "adulterated, counterfeit or otherwise ineffective" drugs could be used in executions in violation of the ban on cruel and unusual punishment.

State prison officials have gone to great lengths to find the drug amid a severe shortage of sodium thiopental, a sedative that was once used by nearly all 34 death penalty states. The shortage started last year after Hospira Inc., the sole U.S. manufacturer of the drug and the only sodium thiopental maker approved by the Food and Drug Administration, stopped making it.

The DEA took Georgia's stockpile of the drug in March after defense attorneys questioned whether the state properly registered with the DEA before importing it from London. Regulators have since taken stockpiles of the drug from Alabama, Kentucky, South Carolina and Tennessee. Arizona, Arkansas, California, Nebraska and South Dakota also received overseas shipments of the drug.

A Justice Department spokeswoman declined to comment on the probe.

The DEA documents suggested that the investigation ramped up in late March. An official on March 28 distributed an Associated Press article that documented the extraordinary — and legally questionable — methods prison officials took to obtain the drugs.

Above it was a note that urged the offices to "please do a check of said prisons in your area and see if they have now or in the past requested or received this drug via unauthorized means. Please attempt to confiscate any and all controlled substances."