A video has emerged showing an El Salvadorean immigrant armed with a knife charging at a police officer near Boston seconds before the cop shot him dead.

Mario Mejia Martinez, 48, was gunned down in Everett, a town in Massachusetts not far from Boston on April 21.

Mr Martinez's brother Jose Joaquin, 37, said he did not believe he was threatening the officer.

But the district attorney has released new surveillance video which would seem to support the officer's claim.

The video shows the suspect, Mario Mejia Martinez, approaching a police officer who then opens fire on him.

The DA also isolated a picture from one of the cameras they say shows Mejia Martinez holding a knife when he approaches the officer.

Police later recovered the knife from the scene.

The officer points his gun at the suspect (pictured, left)

Mr Martinez then appears to charge at the officers

The district attorney's office said police confronted Mario Mejia Martinez after receiving a report about a man harassing pedestrians in downtown Everett.

Mr Martinez said he did not see his older brother, Mario Mejia Martinez, holding a weapon or advancing toward the officer during the confrontation in Everett Square.

He said the video shows his brother attempting to walk away from the officer.

Mr Martinez told a Spanish-language TV channel: 'My brother did not threaten the police. That's a lie.'

Mr Martinez said: 'My brother was standing there. My brother was not doing anything to anyone.'

The victim Mejia Martinez had no criminal record locally.