The number of death row inmates executed in the United States this year declined to its lowest since 1991, according to an annual report released Wednesday by Death Penalty Information Center (DPIC). Death sentences issued in the U.S. also dropped to a low since 1973, with 49 inmates receiving death sentences this year compared to 73 sentences in 2014.

The DPIC report stated that 28 inmates were put to death in six states in 2015 -- far below the peak of 98 in 1999 and lower than the 35 executions last year. The report also showed that there were fewer death sentences in the past 10 years than in the decade before the 1972 Furman v. Georgia case, when the Supreme Court ruled the country’s death penalty statutes unconstitutional.

“Executions were put on hold or remained on hold in many states, partly because of the difficulties in obtaining lethal injection drugs or in establishing acceptable protocols for lethal injections. The governor of Pennsylvania joined governors in Washington, Oregon, and Colorado in declaring moratoria on executions in their states, and a new governor in Oregon agreed to continue the moratorium in that state,” the DPIC said, in the report.

Eighteen states with death penalty -- including Georgia, Missouri and Virginia, which carried out executions in 2015 --  did not issue death sentences this year. 

The last inmate to be executed this year was Brian Keith Terrell, on Dec. 9 in Georgia. Terrell was convicted of forging checks of his mother’s friend and killing him after he was asked for the money.

“The use of the death penalty is becoming increasingly rare and increasingly isolated in the United States,” Robert Dunham, DPIC executive director, said, according to Time magazine. “These are not just annual blips in statistics, but reflect a broad change in attitudes about capital punishment across the country.”

Six death row inmates were exonerated in 2015, according to the report. Since 1973, 156 men and women from 26 states have been absolved of execution.