The death penalty in the United States continued its pattern of broad decline in 2013, with experts attributing the low numbers 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 American courts this year, compared with a peak of 315 in 1994, and 39 executions took place, compared with 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.