Boston Marathon bombing
Officer Dennis Simmonds died a year after sustaining injuries at the Watertown shooting following the Boston Marathon bombing.REUTERS/Brian Snyder

A Boston police officer has been declared the fifth death attributed to the Boston Marathon bombers Dzhokhar and Tamerlan Tsarnaev.

Officer Dennis Simmonds's family will receive line of duty death benefits, the State Retirement Board announced on 28 May.

Officer Simmonds, who died almost a year after taking part in the Watertown shooting with the two bombers, suffered injuries that ultimately lead to his death, medical evidence showed. "His injuries were persistent after the episode involving the gun battle between police and the Boston Marathon bombers on 4/18/2013 in Watertown," a doctor wrote in a medical panel report.

The 28-year-old's younger sister, Nicole Simmonds, told reporters he was knocked to the ground after sustaining head injuries he never truly recovered from.

Using a note from the Boston Police Department as reference the same doctor said the officer "was involved in a shootout and he was crouched for a long period of time. Once the scene was cleared, he went to St Elizabeth's Hospital" by ambulance.

WBZ-TV reported Simmonds's colleague Patrick Rose witnessed his injuries the night of the Watertown shooting. "I drove Dennis out of Watertown, I believe his injuries were sustained that evening," Rose said.

Family will receive almost £100,000 in compensation

Simmonds collapsed and died at the Boston Police Academy in April 2014

According to WCVB, the State Retirement Board approved a one-time $150,000 (£98,000) benefit for his family.

"We absolutely want him to be recognised as the fifth victim," Nicole Simmonds said. "It's now etched in stone, it's etched in paper that my brother is recognised as a hero among those key decision makers."

While the board did not call Simmonds the fifth marathon bombing victim, it noted the report from the state medical panel found his injuries were consistent with the events during the shooting. Boston Mayor Marty Walsh said he hoped the benefit brings comfort to the Simmonds family.

"It doesn't bring him back, but I hope it gives them peace and a little bit of happiness in their heart knowing he died doing the job he loved," Walsh said.