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Boston bombs 'trigged remotely'

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Boston bomb suspects Tamerlan Tsarnaev, left, and brother Dzhokhar Tsarnaev (AP/The Lowell Sun and Robin Young)

Boston bomb suspects Tamerlan Tsarnaev, left, and brother Dzhokhar Tsarnaev (AP/The Lowell Sun and Robin Young)

AP/Press Association Images

People attend an interfaith service held near a makeshift memorial on Boylston Street, near the finish line of the Boston Marathon (AP)

People attend an interfaith service held near a makeshift memorial on Boylston Street, near the finish line of the Boston Marathon (AP)

AP

Officers at the scene of Dzhokhar Tsarnaev's arrest in Watertown (AP)

Officers at the scene of Dzhokhar Tsarnaev's arrest in Watertown (AP)

AP

Stephanie Prashad, the first person escorted back onto Boylston Street, stands in her apartment near the Boston Marathon finish line (AP)

Stephanie Prashad, the first person escorted back onto Boylston Street, stands in her apartment near the Boston Marathon finish line (AP)

AP

Zubeidat Tsarnaeva, mother of Tamerlan and Dzhokhar Tsarnaev, near her home in Makhachkala, Dagestan, southern Russia (AP)

Zubeidat Tsarnaeva, mother of Tamerlan and Dzhokhar Tsarnaev, near her home in Makhachkala, Dagestan, southern Russia (AP)

AP

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Boston bomb suspects Tamerlan Tsarnaev, left, and brother Dzhokhar Tsarnaev (AP/The Lowell Sun and Robin Young)

The Boston Marathon explosions which killed three people and injured more than 260 were triggered by a remote-controlled detonator, US officials have said.

They said the bombs were not very sophisticated. One described the detonator as "close-controlled" - meaning it had to be triggered within several blocks of the bombs.

It was not immediately clear what the detonation device was.

A criminal complaint outlining federal charges against the surviving bombing suspect, Dzhokhar Tsarnaev, described him as holding a mobile phone in his hand minutes before the first explosion.

Mobile phones have been used to trigger bombings in war zones.

PA Media