The Boston Globe, citing an official with knowledge of the investigation, reported that one suspect in Boston’s Marathon bombings had been captured, while another remains on the loose in Watertown after exchange of gunfire with police officers.

A violent incident erupted in Watertown late Thursday night and early Friday morning when shooting killed a police officer.

Police descended on a vehicle in the neighbouring Watertown community later in the evening amid reports of gunfire and explosions, the ABC TV station in Boston reported.

The FBI has not confirmed a connection between the events in Watertown to the twin explosions at the Boston Marathon on Monday.

The FBI had earlier released images and videos of two men wanted as key suspects in connection to the marathon bombing that killed three people and left at least 170 people injured.

The FBI has appealed for the public’s help in their capture.

Meanwhile, President Obama warned on Thursday that “whichever terrorists are behind the attack on Monday picked the wrong city as a target, because Boston will not be terrorized or intimidated.”

On Friday, 1 February the suspects were arrested by the police in relation to allegations of their participation in a bank-robbery in Kozani, in the north of the country.

Police have said the four were injured during their arrest and that only necessary force was used, but family members and lawyers allege that there was no clash, and that the young men were severely beaten in detention.

“We are profoundly concerned about the recurring allegations of torture and ill-treatment by Greek police,” said Marek Marczyński, Deputy Director of the Europe and Central Asia Programme at Amnesty International.

Video and unedited pictures published by media show extensive bruising. However, the police released photographs that had been crudely edited to lighten or erase bruises and cuts.

In one case a suspect’s face in his photograph was unnaturally flattened by the removal of bruising. In another, the photograph was lightened so much that the suspect’s hair turned from brown to blond.

Despite this, the Minister for Public Order said that the editing was necessary to allow for the suspects to be recognized.

“The Greek authorities cannot just Photoshop their problems away. This culture of impunity needs to be stopped,” said Marczyński.

“There has to be an effective, thorough and impartial investigation into allegations of police torture. Such an investigation has to identify those responsible and they should promptly be brought to justice.”

Two of the four detainees are accused of being members of the armed group ‘Conspiracy of the Cells of Fire’.

In October last year 15 anti-fascist demonstrators were allegedly tortured by police following a demonstration.