U.S. prosecutors will seek the death penalty against the suspect in last year's deadly Boston Marathon bombing.

The top American legal official, Attorney General Eric Holder, announced Thursday that the government would ask for the execution of 20-year-old Dzhokhar Tsarnaev if he is convicted. Tsarnaev is accused of detonating two home-made bombs at the annual race last April that killed three people and injured more than 260 others.

The federal government rarely seeks the death penalty, and has only executed three killers since 1988, when a moratorium on executions ended. Holder said the "nature of the conduct at issue and the resultant harm" from the Boston bombings compelled him to seek Tsarnaev's execution.

Prosecutors allege that Tsarnaev, then 19, and his brother, 26-year-old Tamerlan Tsarnaev, two ethnic Chechens who lived in the U.S. for a decade, built and planted two pressure-cooker bombs near the finish line of the race to retaliate against U.S. military action in Muslim countries.

No date has been set for Tsarnaev's trial, but he faces 30 charges, including use of a weapon of mass destruction.

Tamerlan Tsarnaev was killed in a shootout with police last year, soon after the bombings.

After a massive manhunt, police found Dzhokhar Tsarnaev hiding in a boat parked in the backyard of a suburban Boston home.

Authorities allege that he scrawled several messages on the inside of the boat, including one that said, "We Muslims are one body, you hurt one you hurt us all."