Boston Police Commissioner William Evans said the man, identified as Usaama Rahim, had refused to throw down a military-style knife. He had been told to do so several times and continued to approach them.

Officers in a Joint Terrorism Task Force, comprising a police officer and an FBI agent, had then done "what they are trained to do," Evans said.

"The Boston Globe" newspaper said authorities had been searching a nearby apartment building. Officials did not say why Rahim was under surveillance.

"He was someone we were watching for quite a time," Evans said.

Tuesday's fatal shooting coincided with mounting debate in the United States over the use of deadly force by police, especially against minorities.

Group vows to monitor investigation

A spokesman for the Council of American-Islamic Relations (CAIR) identified Rahim's brother, Ibrahim, who in a Facebook entry said: "He was on his cell phone with my dear father during the confrontation needing a witness."

CAIR spokesman Ibrahim Hooper said the council would monitor the investigation.

"We have a number of questions: Why exactly was he being followed? What was the probable cause for this particular stop? Were there any video cameras or body cameras of the incident? How do you reconcile the two versions of the story, the family version being that he was on his normal commute to work at a bus stop?" Hooper said.

News agency Associated Press said Boston voter registration records for Usaama Rahim listed him as a student.

According to Commissioner Evans, the police officer and FBI agent were being evaluated in hospital for "stress."

Deadly police force under scrutiny

Two major newspapers, the "Washington Post" and "The Guardian," have begun their own counts of people killed by police amid public frustration over a lack of comprehensive official data nationwide.

The debate escalated last August over the fatal shooting of unarmed black teenager Michael Brown in Ferguson, Missouri, an event that touched off weeks of protest.

The Post, outlining its initial findings last weekend, found that police across the US had shot and killed persons at a rate of more than two a day.

ipj/cmk (AP, AFP)