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A five-month investigation by The Boston Globe has provided new insight into the history and character of Tamerlan and Dzhokhar Tsarnaev, the brothers accused of carrying out the Boston Marathon bombings in April this year, including suggestions that Tamerlan was dealing with mental illness.

The allegation was previously suggested in another major profile piece by Rolling Stone, but the Globe goes deeper into that proposition. Anna Nikaeva, a Chechen living in the Newton area, said that Zudeibat Tsarnaev, the family matriarch, didn't let herself see the possibility of her favourite Tamerlan's sickness.

He had told his mother that he felt there were two people living inside of him. I told her, ‘You should get that checked out.’ But she just said, ‘No, he’s fine.’ She couldn’t accept the tiniest criticism of him. But obviously she was thinking about it enough that she brought it up.

Don Larking, a friend who went with Tamerlan to mosque, said that after they became close, Tamerlan confided about the voices in his head, suggesting that it was "majestic mind control":

'You can give a signal, a phrase or a gesture, and bring out the alternate personality and make them do things. Tamerlan thought someone might have done that to him.' The person inside him, as Tamerlan described it to Larking, 'was someone who wanted to control him to make him do something.'

After Tamerlan returned to the U.S. after some time in his native Dagestan, Larking said he became more serious, and the last time he spoke about the voices, it was in fearful tones.

Tamerlan, a former boxer, was killed in a shootout in Watertown, and some called for his brain to undergo study to check for psychosis or trauma. The Globe says it appears that Tamerlan's family did not pursue treatment for him.

Other revelations from the remarkable long-read include:

A drug-dealing Dzhokhar should have flunked out of school, but did not.