Media playback is unsupported on your device Media caption Superintendent Timothy Alben: "This is the second step of many that you will see over months and further"

Boston Marathon bombing suspect Dzhokhar Tsarnaev has been formally charged with killing four people and using a weapon of mass destruction.

Mr Tsarnaev, 19, faces 30 total charges in the 15 April blasts, which killed three and injured more than 260 others.

A fourth victim, a policeman, was shot dead by him and his brother as they fled authorities, prosecutors say.

If convicted, Mr Tsarnaev faces life in prison or even the death penalty, US prosecutors said.

In Boston, US Attorney Carmen Ortiz described the Tsarnaev brothers' preparation for the attack, which she said began more than two months earlier.

'Civilians killed'

Dzhokhar Tsarnaev and his brother Tamerlan, 26, went to a firing range to take target practice, bought electronic material online that could be used to make bombs, and downloaded a publication that could provide instructions on building explosives, Ms Ortiz told reporters.

Dzhokhar Tsarnaev also faces charges for a carjacking days after the attacks and for interfering with commerce, prosecutors said.

Tamerlan Tsarnaev was killed in a shoot-out with police days after the twin blasts. Dzhokhar Tsarnaev was injured in the manhunt and has been held in a prison hospital near Boston since his capture on 19 April.

Last month, the suspect's mother said her son had recovered enough to walk, in an interview with the Associated Press. But officials declined to comment on his condition on Thursday.

Dzhokhar Tsarnaev was found hiding in a boat parked in a residential garden in Watertown, Massachusetts, where according to the indictment he wrote: "The US Government is killing our innocent civilians" and "I can't stand to see such evil go unpunished."

According to the charging document, the brothers made bombs from pressure cookers, low-explosive powder, ball bearings, nails, adhesive, electronic components and other material.

It adds that Dzhokhar Tsarnaev aided and abetted his brother by planting and detonating one of the bombs.

Ms Ortiz said at the briefing that she had met the families of those killed and wounded in the attack.

"Their strength is extraordinary and we will do everything that we can to pursue justice not only on their behalf, but on the behalf of all us," she said.

The brothers are from a family of ethnic Chechen Muslims from Russia and had been living in the US for about a decade.