New York (AFP) - An official report into the aftermath of the 2013 Boston Marathon bombings has concluded US police lacked "weapons discipline" while hunting down the two chief suspects behind the attacks.

The Massachusetts state government report said weapons discipline was a problem during two shootouts which left one suspect dead and ended with the capture of the other, who is now on trial.

The April 15, 2013 double bombing of Boston's famed marathon killed three people and wounded 264 in the worst attack on US soil since 9/11.

The report said more than 200 rounds of ammunition were expended on April 19 during a first firefight that left Tamerlan Tsarnaev -- also run over by his own brother -- fatally wounded.

Officers opened fire "without necessarily having identified and lined up their target or appropriately aimed their weapons," it said.

An unmarked black pickup truck -- driven by a law enforcement official -- was also erroneously reported as stolen and fired on.

After police were tipped off that second suspect Dzhokhar Tsarnaev was hiding in a boat, another officer opened fire "without appropriate authority" in response to a perceived movement.

That caused "many officers to fire at the boat in the belief that they were being shot at by the suspect. Each of these incidents created dangerous crossfire situations," the report said.

After a nearly two-hour standoff the suspect was arrested and transported by ambulance under heavy guard to hospital.

Dzhokhar Tsarnaev faces the death penalty if convicted by a jury, expected to start deliberations next Monday.

If found guilty, the second stage of the trial will determine whether he would be sentenced to death or life without parole.