NASHVILLE (AP) — Heavily redacted email records from the state Department of Correction raise questions about access to certain execution drugs, according to a federal court filing Tuesday by an attorney for death row inmates.

The emails mention the state's "difficult time" getting its hands on the paralytic used as the second in Tennessee's three-drug series, vecuronium bromide.

One email mentions that the compounded heart-stopping third drug, potassium chloride, at one point experienced an issue that the attorney wrote would be akin to "injecting rocks into the veins."

The department said the issue had been occurring in an Aug. 8 email and did not tell three men before letting them choose the electric chair, effectively waiving their legal rights to challenge lethal injection or electrocution, Supervisory Assistant Federal Public Defender Kelley Henry wrote in the filing.

Stephen West died in the electric chair in August as did Lee Hall last month. Nicholas Sutton is slated for execution Feb. 20. Additionally, the state did not mention any issue with the drug before Donnie Johnson was put to death by lethal injection in May, a few months before the state's email confirmed the problem had been occurring, the filing states.

And since the state had no known source of vecuronium bromide by Nov. 5, if Hall had not chosen that day to be electrocuted, the state "would have been forced to inform him that they could not carry out his execution by lethal injection, paving the way for Mr. Hall to challenge the constitutionality of Tennessee's electric chair," the filing states.

Another email says there may be a "loop hole" to getting another drug previously used by Tennessee on its own, pentobarbital, by importing it. The filing contends the state refuses to return to pentobarbital, "without a legitimate reason, even though it would significantly reduce a substantial risk of severe pain."

The court filing comes at a time when Tennessee is killing more inmates than any state but Texas, and five of the last seven people executed in Tennessee have chosen to die in the electric chair instead of by lethal injection.

Attorneys for inmates have contended both methods result in an unconstitutionally painful death, but some have contended it would be quicker and less painful to die from electrocution.

Three more executions are scheduled this year and the state Supreme Court is considering whether to schedule seven others.

The compounding issue with the heart-stopping drug was raised in an Aug. 8 email. The availability of the paralytic was discussed in a Nov. 21 email, which raised the possibility of finding a different drug and changing the lethal injection protocol.

The filing says the state's supply of manufactured vecuronium bromide had expired by Nov. 5, adding that it does not appear the state has secured a new supply of it or another paralytic.

And in an Oct. 30 email, the possibility of obtaining pentobarbital was discussed.

"It's possible we could order it but getting it imported would be the issue," the email states. "It's a schedule II drug and the DEA has already advised us that they do not allow the importation of drugs that are considered 'readily available' in the US. There may be a loop hole there given that the product is technically not 'readily available.'"

Names, email addresses and large sections of messages are redacted in the records.

The filing says a recent opinion from the Office of Legal Counsel now allows for the importation of pentobarbital from foreign sources. Other states have used the drug as recently as last month; and the federal government intends to use it.

In responses, the state has said those factors don't make the drug available to Tennessee for executions. The state also said attorneys for death row inmates had their chance to argue their case in a failed state court challenge of Tennessee's execution protocol.

"If foreign manufacturers become a plausible source of pentobarbital for states in the wake of the OLC opinion, it stands to reason that anti-death-penalty advocates will renew their efforts to cut off that source as well," the state wrote in a recent filing.