ST. LOUIS -- Eighteen convicted killers have been executed in Missouri in the last two years -- a pace that is expected to slow, with death sentences on the decline and many of the 28 remaining death-row inmates still filing appeals.

Only Texas, with 24, has performed more executions than Missouri since November 2013. But execution appears to be imminent for just one Missouri inmate.

"They've basically run out of people to kill, to put it in an undiplomatic way," said Kent Gipson, a Kansas City-based attorney who represents several inmates.

Executions across the U.S. largely were on hold for many years because of a de facto moratorium as the Supreme Court grappled with the constitutionality of lethal injection. Once that cleared, execution drugs became hard to obtain because major companies refused to sell drugs for lethal use.

Missouri turned to pentobarbital, obtained from a source the state won't disclose, in late 2013. By then, there was a backlog of inmates whose appeals were exhausted. Attorney General Chris Koster asked the state Supreme Court to set execution dates for many of them, and the court obliged, scheduling one a month since November 2013 (some were halted by court action or clemency).

At the same time, death sentences have declined for many years. Nationally, the 72 death sentences issued last year were the fewest since 1976, according to Amnesty International. None was issued in Missouri.

Death-penalty expert Deborah Denno, who teaches at Fordham Law School, cited several factors, including concern over the availability of lethal drugs and worries about executing the innocent.

"I think juries aren't sending people to death as much, and I think prosecutors aren't bringing them up as often," Denno said.

Execution could be near for Earl Forrest, convicted of killing a Dent County sheriff's deputy and two others during a 2002 crime spree. Forrest is out of appeals, and Koster has requested an execution date.

But 16 of the remaining 26 Missouri inmates haven't exhausted their appeals. Another two have claimed innocence, and their cases are being reviewed. Two others technically on death row have been ruled mentally incompetent for execution.

Medical conditions have cast doubt in two cases this year. Ernest Lee Johnson was scheduled to die last week for killing three convenience store workers in 1994, but the U.S. Supreme Court sent the case back to an appeals court for review because his attorneys argued the execution drug could cause violent seizures because of a benign brain tumor.

In May, Russell Bucklew, convicted of killing a Southeast Missouri man during a 1996 crime spree, was granted a stay over concerns the execution drug could cause suffering because of a congenital condition that includes weakened and malformed blood vessels and nose and throat tumors.

Courts have ordered new sentencing hearings for three inmates over attorney and procedural errors, and the U.S. Supreme Court remanded one case to the district court over attorney competence concerns because a federal appeal was not filed on time.

William Boliek is likely to live out his life on death row. His execution was stayed by Gov. Mel Carnahan. Carnahan died in a plane crash without acting on it, and a ruling determined only Carnahan could overturn the stay.

A spokesman for Gov. Jay Nixon has said William Boliek will not be executed.