US authorities failed to detain Tamerlan Tsarnaev, who would later become one of the two Boston Marathon bombers, when he entered the country in 2012, because of a spelling mistake in a security database, a Congressional report has found.

According to the report, which was prepared by the House Homeland Security Committee, the FBI and the CIA had been warned of Tsarnaev’s extremist views by Russian authorities in 2011. After an investigation and a personal interview, the CIA concluded that Tsarnaev had no links to terrorism. Nevertheless, the agency entered Tsarnaev’s name, in its correct spelling, into a Customs and Border Protection database, NBC reported Wednesday.

But when they refreshed the blacklist after the investigation, CIA officials accidentally misspelled Tsarnaev’s name as “Tsarnayev.” Thus, his name did not alert security officials when he returned from a 2012 trip to Russia, where, according to US officials, he “may have been in touch” with suspected militants.

In the wake of the bombing, Homeland Security Secretary Janet Napolitano acknowledged that the system had “pinged” when Tsarnaev left the US, but said his re-entry remained unnoticed.

“By the time he returned all investigations had been closed,” she said at the time.

Three people were killed and some 260 were wounded on April 15 last year when two explosives-packed pressure cookers exploded near the finish line of the Boston Marathon. Several of the injured lost limbs.

Tsarnaev, then 26, and his 19-year-old brother, Dzhokhar, were cornered by police after a four-day manhunt. Tamerlan died after an exchange of fire with police while Dzhokhar was wounded.

In January, US Attorney General Eric Holder said that courts would seek a rare federal death penalty for Dzhokhar Tsarnaev.

“The nature of the conduct at issue and the resultant harm compel this decision,” Holder said in a statement on the prosecution of the now-20-year-old, a US citizen from a Chechen Muslim family.

The brothers are said to have built the bombs with help from an online al-Qaeda magazine, but they are not accused of having received help from any organized foreign terror group.

If Tsarnaev is executed, he will be the first defendant to be put to death at federal level since Timothy McVeigh, who went to the death chamber in June 2001 for the Oklahoma City bombing.

AFP contributed to this report