Major public events in the US such as the Boston Marathon need greater security, but officials should avoid creating a "police state", Boston's police commissioner told a congressional hearing today.

Edward Davis called for more use of surveillance cameras and other technology, as well as special police units and more undercover officers, as ways to provide tighter security.

However, he said that the public's privacy must also be protected.

"We do not, and cannot, live in a protective enclosure because of the actions of extremists who seek to disrupt our way of life," Commissioner Davis said in prepared testimony delivered before US politicians probing the 15 April bombings.

Three people were killed and 264 injured in the attack.

The hearing by the US House of Representatives Homeland Security Committee is the first in a series examining events leading up to the bombings.

Surveillance video aided US officials in naming two ethnic Chechen brothers as responsible for the attack.

One, Dzhokhar Tsarnaev, was captured and charged in the deadly bombing. 

His older brother, Tamerlan, died after a shootout with police.

Federal authorities have also charged three others with interfering with the investigation.

The probe is continuing, with the focus now on computer evidence as well as a trickle of information coming from Russian authorities.

Commissioner Davis told politicians that surveillance images can be helpful to police as evidence and are aimed at protecting the public, not stifling freedom or chilling free speech.

Still, he added: "I do not endorse actions that move Boston and our nation into a police state mentality, with surveillance cameras attached to every light pole in the city."

Committee chairman Michael McCaul has said his panel will focus on how US efforts have evolved to address threats such as the bombing that are rarely seen in the US but are more common in other countries.

Politicians will "examine the interaction of state and local authorities with federal law enforcement prior to and after the attack, and what, if anything, they were told about intelligence regarding the suspects", he said.

"Ultimately, we hope to find out whether intelligence was shared, and what we need to improve to prevent another terrorist attack on our homeland."

Boston suspect's widow faces further questioning

Tamerlan Tsarnaev's widow still faces questions from federal authorities and has hired a criminal lawyer with experience defending terrorism cases.

Katherine Russell added New York lawyer Joshua Dratel to her legal team, her attorney Amato DeLuca said last night.

Mr Dratel has represented a number of terrorism suspects in federal courts and military commissions, including Guantanamo Bay detainee David Hicks, who attended an al-Qaeda-linked training camp in Afghanistan.

Mr Dratel's "unique, specialised experience" will help ensure that Ms Russell "can assist in the ongoing investigation in the most constructive way possible," Mr DeLuca said in a written statement.

Tamerlan and Dzhokhar Tsarnaev, ethnic Chechen brothers from southern Russia living in Massachusetts, are accused of planting two shrapnel-packed pressure-cooker bombs near the marathon finish line.

Mr DeLuca said Ms Russell, who has not been charged with any crime, will continue to meet with investigators as "part of a series of meetings over many hours where she has answered questions".

Mr DeLuca and Miriam Weizenbaum have been representing Ms Russell, who is from Rhode Island.

They specialise in civil cases such as personal injury law.

An FBI spokeswoman would not comment when asked if Ms Russell is cooperating.

Mr DeLuca has said Ms Russell had no reason to suspect her husband and his brother in the deadly bombing.

Dzhokhar Tsarnaev, who was captured hiding in a boat outside a house in a Boston suburb, was charged with using a weapon of mass destruction to kill.

Their mother has said the charges against them are lies.

Suspect's body buried

Tamerlan Tsarnaev’s body has been entombed and is no longer in the city of Worcester, Massachusetts, where it had been held at a funeral home, the Worcester Police Department said this afternoon.

The police did not disclose where the body had been moved.

The question of where to bury his body had proven controversial, with city officials in Boston and in Cambridge, Massachusetts, where he had lived, refusing to accept the body for burial.

A crowd had picketed outside the Worcester funeral home where the body had been held since it was claimed from the medical examiner last week.

"A courageous and compassionate individual came forward to provide the assistance to properly bury the deceased," Worcester police said in a statement posted on the department's website.