Media playback is unsupported on your device Media caption The brothers "planned more attacks", Ian Pannell reports from Boston

The brothers suspected of bombing the Boston Marathon were probably planning further attacks, the city's police commissioner has said.

Ed Davis told CBS News that Dzhokhar and Tamerlan Tsarnaev had been carrying homemade bombs and grenades which they threw at police when cornered.

A top US interrogation group is waiting to question Dzhokhar Tsarnaev, who is in a serious condition in hospital.

His elder brother died during a gunfight with police on Thursday night.

Analysis The FBI had been warned that the man who apparently carried out the first terrorist attack on an American city since 9/11 was a strong supporter of radical Islam. People will want to know how far they delved, how hard they tried, how seriously they took the information. Some of the criticism will be unfair, based on hindsight - they must get thousands of such warnings ever year. Or perhaps they are quite rare. That is another question. America has been in a kind of limbo while the identity of the bombers was unknown. With greater certainty the questions, and the politics, will begin.

Two women and an eight-year-old boy were killed in Monday's blasts, close to the finish line of the marathon.

A police officer was killed and a transport officer shot in the thigh during the operation to track down the brothers.

Doctors treating the wounded officer, Richard Donohue, said on Sunday he was in a stable but critical condition.

The pair clashed with police on Thursday night, in the shootout that killed the elder brother.

Speaking to CBS's Face the Nation, Mr Davis said: "We have reason to believe, based upon the evidence that was found at that scene - the explosions, the explosive ordnance that was unexploded and the firepower that they had - that they were going to attack other individuals."

"That's my personal belief at this time."

He said more than 250 rounds of expended ammunition were found at the scene, and that the ground was "littered with unexploded improvised explosive devices that we had to point out to the arriving officers".

Media playback is unsupported on your device Media caption Boston police chief Ed Davis said improvised devices were strewn around the area

Another device was found inside a car the brothers had earlier hijacked.

Officials were now trying to trace all the weapons used by the brothers, he said, adding that this would be a "significant part of the investigation".

'Unable to speak'

Dzhokhar Tsarnaev escaped the clashes with police, but was arrested late on Friday when he was found seriously injured and hiding inside a boat in a suburban backyard.

He is under armed guard at the Beth Israel Deaconess Memorial Hospital, where many of the bomb victims are also being treated - he is heavily sedated and has a breathing tube in his throat.

The Tsarnaev brothers Sons of Chechen refugees from the troubled Caucasus region of southern Russia

Family is thought to have moved to the US in 2001, from Russian republic of Dagestan

They lived in the Massachusetts town of Cambridge, home to Harvard University

Dzhokhar, 19, (right) was awarded a scholarship to pursue further education; he wanted to become a brain surgeon, according to his father

Tamerlan, 26, was an amateur boxer who had reportedly taken time off college to train for a competition; he described himself as a "very religious" non-drinker and non-smoker Profile: The Tsarnaev brothers Q&A: Suspects' Chechen links Maps of manhunt Timeline of manhunt

The High-Value Detainee Interrogation Group - a multi-security agency unit specialising in questioning terror suspects - are waiting to question him in the hope he will give some clue as to his motive and whether the pair had outside help.

But he has not yet been able to speak and Boston's Mayor Tom Menino told ABC News on Sunday: "We don't know if we'll ever be able to question the individual."

Prosecutors have not yet determined what charges the teenager might eventually face.

A federal charge of using a weapon of mass destruction to kill people carries a possible death sentence. There is no death penalty in the state of Massachusetts.

'Brainwashed'

The two bombs - placed inside pressure cookers packed with shrapnel and hidden in backpacks - exploded amid crowds standing close to the finishing line of the Boston Marathon on Monday afternoon, killing three people.

Media playback is unsupported on your device Media caption Boston mayor Tom Menino: "We don't know if we'll ever be able to question the individual"

More than 170 people were injured, of whom more than 50 are still in hospital, three in a critical condition.

Mr Menino said evidence indicated that the accused pair had acted alone, but that the elder of the two, 26-year-old Tamerlan Tsarnaev, had "brainwashed his younger brother" into carrying out the attack.

The governor of Massachusetts, Deval Patrick, said video surveillance footage firmly placed Dzhokar Tsarnaev at the scene of the first explosion.

"It does seem to be pretty clear that this suspect took the backpack off, put it down, did not react when the first explosion went off and then moved away from the backpack in time for the second explosion,'' he told NBC News, saying he had been briefed on the footage by law enforcement officials.

Suspects' origins Chechnya (pair were ethnic Chechens): Southern Russian republic, rich in oil. Infrastructure hit by years of war between separatists and Russian forces, banditry and organised crime. Improved security situation has led to increased investment in reconstruction projects. But sporadic attacks by separatists continue. Dagestan (pair lived here for several years): Southern Russian republic, translating as "land of the mountains", famed for ethnic and linguistic diversity. A long-running militant Islamist insurgency is a thorn in the authorities' side. Dagestan has oil reserves and a strong manufacturing sector, but rampant corruption and organised crime . Dagestan profile Chechnya profile

"It's pretty clear about his involvement and pretty chilling, frankly," he said.

On Saturday, Governor Patrick had told reporters that he hoped Dzhokhar Tsarnaev survived "because we have a million questions, and those questions need to be answered".

The Tsarnaev brothers are ethnic Chechens who had been living in America for about a decade.

One key line of inquiry into the motives behind the attack will be a six-month trip made by Tamerlan Tsarnaev to Dagestan in the Russian Caucasus in 2012.

The FBI had interviewed Tamerlan Tsarnaev in 2011 after a request from a foreign government, US law enforcements officials have confirmed.

But agents closed the case after finding no cause for concern.

Dagestan has had a long-running Islamist insurgency, but a prominent militant group in the region, the Mujahedeen of the Caucasus Emirate Province of Dagestan, denied any link to the Boston attacks, saying in a statement it was not fighting the US but Russia, and did not attack civilians.

Several members of the Tsarnaev family have condemned and disowned the brothers, but their parents have insisted they must have been framed.