Dennis McGuire was put to death on Thursday in an execution that lasted 26 minutes. Handout via Getty Images

Prison authorities in two states are facing increased pressure to halt executions immediately after a controversial execution last week using a previously untested drug mix left a condemned man, Dennis McGuire, gasping and snorting for close to half an hour.

The American Civil Liberties Union (ACLU) wrote to Ohio's governor, John Kasich, Sunday urging him to use his executive authority ahead of five scheduled executions in 2014 after McGuire's execution on Thursday.

It took McGuire 26 minutes to die after the chemicals — intravenous doses of the sedative midazolam and the painkiller hydromorphone — began flowing into his body. McGuire was sentenced to die for raping and stabbing a pregnant newlywed to death in 1989, and his death was the longest execution of the 53 carried out in Ohio since capital punishment resumed 15 years ago.

Meanwhile, another group, the ACLU of Montana, is now suing Montana overs its execution method, which last year changed from a combination of three drugs to two.

Though the drugs used in the Ohio execution are different from the ones called for in Montana's execution protocol, critics say the Ohio case illustrates the dangers of using untested drug combinations.

No one has been executed in Montana since the state changed its lethal-injection method, and its two-drug combination has not been tested in the U.S., said ACLU of Montana attorney Anna Conley.

Executions should not be carried out without better knowledge of how the chemicals involved will work together, she said. "Much like Ohio, the two-drug protocol Montana would like to use has never been tested. We have no idea how it will work."