The Justice Department has agreed to seek the federal death penalty in at least two murder cases, in what officials say is the first sign of a heightened effort under Attorney General Jeff Sessions to use capital punishment to further crack down on violent crime.

In a decision made public Monday, Mr. Sessions authorized federal prosecutors to seek the death penalty against Billy Arnold, who is charged with killing two rival gang members in Detroit.

The decision followed the first death-penalty authorization under Mr. Sessions, made public Dec. 19, when he cleared prosecutors in Orlando to seek a death sentence against Jarvis Wayne Madison, who is charged with fatally shooting his estranged wife in 2016.

The Justice Department is also considering seeking death sentences against Sayfullo Saipov, accused of killing eight people in November by driving a truck onto a Manhattan bike lane, and against two defendants in the 2016 slaying of two teenage girls by MS-13 gang members on Long Island, outside of New York City, according to people familiar with the deliberations.

Mr. Sessions views the death penalty as a “valuable tool in the tool belt,” according to a senior Justice Department official. The official said the death penalty isn’t only a deterrent, but also a “punishment for the most heinous crimes prohibited under federal law.”