Executions in the United States have been on a fairly steady decline in recent years, dropping to 28 last year — the lowest since 1991. A peak of 98 came in 1999.

Difficulty obtaining lethal injection drugs after many manufacturers stopped selling their products for use in executions has made it tough for some states to execute existing death row inmates. Other reasons for the decline include better legal representation for those facing the death penalty, life-in-prison sentences without parole, and the high cost of death penalty prosecutions.

Georgia is set to execute its oldest death row inmate Tuesday. Brandon Astor Jones, 72, was convicted in the 1979 killing of convenience store manager Roger Tackett. Van Roosevelt Solomon, who was also convicted and sentenced to death for the killing, was executed in Georgia's electric chair in February 1985.

Here's a look at some death penalty facts and figures.

WHO ALLOWS THE DEATH PENALTY?

Capital punishment is legal in 31 states. But only six states — Florida, Georgia, Missouri, Oklahoma, Texas and Virginia — carried out executions in 2015. Texas had the most with 13, followed by Missouri with six and Georgia with five.

Michigan has the longest-standing ban on the death penalty; the state did away with capital punishment in 1846. Seven states have abolished the death penalty in the past 10 years: Nebraska (2015), Maryland (2013), Connecticut (2012), Illinois (2011), New Mexico (2009), New Jersey (2007) and New York (2007).

But Nebraska's ban faces a test. Death penalty supporters launched a successful drive to get the issue on this year's general election ballot. And New Mexico's ban wasn't retroactive, leaving two people on death row.

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FALLING NUMBERS

Over the past decade, the number of executions has fallen pretty steadily in the U.S.

2006: 53

2007: 42

2008: 37

2009: 52

2010: 46

2011: 43

2012: 43

2013: 39

2014: 35

2015: 28

The last execution of 2007 was in September in Texas; after that, executions were effectively halted for about seven months while U.S. Supreme Court considered a challenge to a lethal injection method. The court in April 2008 upheld the method, and executions resumed with a May 2008 death in Georgia. That timeline helps explain why the number of executions nationwide dipped more dramatically in 2007 and 2008 and then increased for 2009.

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EXECUTION DRUGS AND SECRECY LAWS

States have scrambled in recent years to find sources of lethal injection drugs after pharmaceutical manufacturers, many of them in Europe, stopped selling their products for use in executions, citing ethical concerns. That has led a number of states to enact laws that shield the identities of their drug suppliers. The states say pharmacies and companies that are willing to make the drugs fear retaliation from death penalty opponents if their identities are made public.

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Information on numbers of executions carried out in each state and years of capital punishment bans comes from the Death Penalty Information Center, which tracks issues related to capital punishment.