LITTLE ROCK — A second expert testified in federal court Tuesday that Arkansas’ planned method for carrying out an unprecedented series of executions this month will cause extreme pain and suffering to the people being put to death.

The testimony came on the second day of a hearing that is expected to last through Thursday in U.S. District Court in Little Rock. Judge Kristine Baker is considering a motion for a preliminary injunction in a lawsuit by nine death-row inmates, seven of whom are scheduled to be executed over 11 days starting Monday.

The state is rushing to kill the inmates before its supply of an execution drug expires at the end of this month.

The inmates allege that the state’s planned lethal-injection method and accelerated execution schedule will violate their constitutional rights, including their right not to be subjected to cruel and unusual punishment. The state denies the allegations.

Arkansas’ plan is to inject each inmate first with the sedative midazolam, then with the paralytic vecuronium bromide, and finally with potassium chloride to stop the heart. On Monday, Dr. Joel Zivot, a professor at Emory University in Atlanta and a doctor specializing in anesthesiology and critical care medicine, testified that the process would be “extremely painful” for the inmates.

On Tuesday, Craig Stevens, a professor of pharmacology at the University of Oklahoma, testified that midazolam, a benzodiazepine, is not a strong enough sedative to render a person unconscious and unable to feel pain. He said that although the state plans to inject each prisoner with a large dose of midazolam — two injections of 250 mg each — his research has shown that at about 228 mg the drug reaches a “ceiling effect,” or a maximum level of effectiveness.

In contrast, Stevens said barbiturates do not have a ceiling effect and said they continue to increase in power well beyond 228 mg. He also said barbiturates have been used as the sole drug in executions, whereas no state has tried to execute anyone with midazolam alone.

“Did you reach a conclusion as to whether you think that the protocol will lead to serious pain and suffering?” asked John Williams, an attorney for the inmates.

“I do, because of the nature of midazolam,” Stevens said.

Midazolam has been used in executions in Oklahoma and Alabama in which inmates appeared to struggle on the gurney. The U.S. Supreme Court upheld the drug’s use in executions in a 5-4 decision in 2015.

The inmates’ suit alleges that barbiturates are a reasonable and preferable alternative to midazolam as a sedative for use in executions. The state has countered that barbiturates are not readily available to it.

During cross-examination of Stevens on Tuesday, Assistant Attorney General Jennifer Merritt noted that a manufacturer of barbiturates said in 2013 it would no longer permit its drugs to be sold to Arkansas for executions.

Merritt also said the state Department of Correction has attempted to obtain barbiturates elsewhere, but “the manufacturer said, ‘No way.’”

“Do you know the identity of any manufacturer or seller of a fast-acting barbiturate that would be willing to sell it to the state of Arkansas to use in a lethal-injection execution?” she asked.

“Personally, no,” Stevens said.

Arkansas has not executed an inmate since 2005 because of legal challenges and difficulty obtaining execution drugs.