The death penalty in the United States continued its pattern of broad decline in 2013, with experts attributing the drop to a critical shortage of drugs used for lethal injection; increasing public concern over judicial mistakes and the expense of capital cases; and a growing preference for life without parole.

Eighty death sentences were imposed by U.S. courts this year, compared to a peak of 315 in 1994, and 39 executions took place, compared to 98 in 1999, according to an annual accounting released on Thursday by the Death Penalty Information Center, a private group in Washington.

"A societal shift is underway," said Richard Dieter, the executive director of the information center, which opposes capital punishment.

In May, Maryland became the sixth state in the last six years to abolish the death penalty, leaving 32 states with capital punishment on the books. But for the second straight year, only nine states put prisoners to death.

Douglas A. Berman, an expert on criminal sentencing at Ohio State University’s Moritz College of Law, said a significant retrenchment in the use of the death penalty was taking place, but also noted that "a majority of states and people still favor using it for the most serious crimes."

Texas, long the nation’s leader in executions, provides strong evidence of a dwindling role for capital punishment. The state carried out 16 executions this year – still the most of any state, but far below the record 40 that it carried out in 2000.

Texas courts sentenced nine defendants to die this year, fewer than the number it executed. It was the sixth year in a row in which fewer than 10 new death sentences were handed down.

Legislatures in Texas and other states have adopted life without parole for severe offenders, which Berman called "the single most important factor in the decline in the death penalty in the last 15 years."

Prosecutors, judges and victims are less likely to demand execution when they know that violent criminals will die in prison, especially in the face of the expense and delays involved in capital cases, Berman said. But he added that civil rights groups now argue that life sentences are imposed too readily on nonviolent offenders as well.

Tangled legal battles over acceptable methods of lethal injection have blocked executions in Arkansas, California and North Carolina for more than seven years. Throughout the country, the issue has been further complicated by shortages of some drugs after European and U.S. manufacturers stopped providing them for executions.

Some states like Ohio and Texas have turned to lightly regulated "compounding pharmacies" to obtain needed drugs. Others, including Florida, propose using new combinations or single-drug protocols, but these plans are in many cases under court scrutiny.

In terms of the future of capital punishment, California may be the biggest unknown. Tied up with legal challenges and the litigation on injection protocols, the state now has 731 prisoners on death row, including some who have been there for three decades.

In 2012, California voters rejected by 52-48 percent a proposal to end the death penalty and give life without parole to those on death row. But they have also elected a governor, Jerry Brown, who opposes capital punishment. Sorting out the lethal injection protocol may still take years, some experts say.

But California courts are still meting out death sentences — 24 this year, more than any other state.

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