A US grand jury has indicted accused Boston Marathon bomber Dzohkhar Tsarnaev on charges of killing four people and using a weapon of mass destruction.

Mr Tsarnaev, 19, is one of two ethnic Chechen brothers accused of carrying out the attack on 15 April, which killed three people.

The indictment includes 30 criminal counts, with 19 of those counts carrying the possibility of the death penalty.

His public defender, Miriam Conrad, declined to comment on the charges.

The pair were accused of setting off two homemade pressure-cooker bombs amid a crowd of thousands of people at the race's finish line.

They lay low for three days after the attack until the FBI released photos of the suspects on 18 April, hoping that the public could identify them.

That night the brothers allegedly set out with five improvised explosive devices, a semiautomatic handgun, ammunition, a hunting knife and a machete.

The pair used those weapons in a battle with police in Watertown, Massachusetts, outside Boston, and Dzhokhar ran over his 26-year-old brother, Tamerlan, with a car in the escape, officials said.

The younger Tsarnaev, who was badly injured in that gun battle, has been held in a prison hospital west of Boston since his capture on 19 April.

The brothers started preparing for the attack more than two months earlier, when Tamerlan Tsarnaev traveled to a New Hampshire fireworks shop to buy 48 mortar shells containing about 3.6kg of explosive powder, according to the charges.

Three people died in the bomb attacks: Krystle Campbell, 29, Lingzi Lu, 23, and eight-year-old Martin Richard.

Tamerlan Tsarnaev had been on a US government database of potential terrorism suspects and the US had twice been warned by Russia that he might be an Islamic militant, according to US security officials.

A congressional hearing after the bombing focused on whether the Federal Bureau of Investigation paid sufficient heed to Moscow, which has been in bitter conflict with Islamic militants in Chechnya and other parts of the northern Caucasus region.

Indictment outlines timeline of events

The indictment follows the pair through the day of the bombing and to the evening of 18 April, after the FBI released pictures of both men at the finish line, in a plea for help from the public in identifying them.

The pair armed themselves with five more homemade bombs, a semiautomatic handgun, a machete and a hunting knife.

They then drove to the Massachusetts Institute of Technology in Cambridge, where they shot their fourth victim. Campus police officer Sean Collier died during their unsuccessful attempt to steal his weapon.

The indictment details how the two carjacked a Mercedes, briefly holding its driver hostage.

They drove to Watertown, where police found them shortly after midnight. The gunbattle followed, with the brothers shooting and throwing bombs at the police.

Dzhokhar Tsarnaev, who had left the car during the battle, jumped back into the Mercedes and drove at the police officers who had been trying to arrest them. In the confusion, he ran over his brother, contributing to his death.

That move allowed Dzhokhar to escape and prompted a day-long lockdown of most of the Boston area.

That evening, a resident found him hiding in a boat in a Watertown backyard, on which he had written messages including: "The US government is killing our innocent civilians" and "I can't stand to see such evil go unpunished," the indictment reads.