Boston Bomb Suspect's Condition 'Fair'

The condition of Boston bomb suspect Dzhokhar Tsarnaev has improved, according to doctors at the medical centre where he is being treated.

The news comes as it was revealed the brothers suspected of bombing the Boston Marathon appear to have been motivated by a radical brand of Islam, but were not apparently connected to any Islamist terrorist groups.

US officials have interrogated and charged the surviving brother with crimes that could bring the death penalty.

The 19-year-old remains in a serious condition with a gunshot wound to his throat and other injuries suffered during his attempted getaway.

His older brother, Tamerlan, 26, died on Friday after a fierce gun battle with police.

Tsarnaev was charged in his hospital bed with using and conspiring to use a weapon of mass destruction.

He was accused of joining with his brother in setting off the shrapnel-packed pressure-cooker bombs that killed three people a week ago.

The brothers, ethnic Chechens from Russia who had been living in the US for about a decade, practised Islam.

Two US officials said preliminary evidence from the younger man's interrogation suggests the brothers were motivated by religious extremism, but were apparently not involved with Islamist terrorist organisations.

Dzhokhar Tsarnaev is said to have communicated with his interrogators in writing due to his injuries.

They said they were still trying to verify what they had been told by him, and were looking at such things as his phone and online communications and his associations with others.

In the criminal complaint outlining the allegations, investigators said Tsarnaev and his elder brother each placed a backpack containing a bomb in the crowd near the finish line of the marathon.

The FBI said surveillance camera footage showed Dzhokhar Tsarnaev manipulating his mobile phone and lifting it to his ear just moments before the two blasts.

After the first explosion, a block away from Dzhokhar "virtually every head turns to the east ... and stares in that direction in apparent bewilderment and alarm", the complaint says.

Dzhokhar Tsarnaev "virtually alone of the individuals in front of the restaurant, appears calm".

He then quickly walked away, leaving a backpack on the ground. About 10 seconds later, a bomb blew up at the spot where he had been standing, the FBI said.

The FBI did not say whether he was using his mobile phone to detonate one or both of the bombs or whether he was talking to someone.

Meanwhile, Kazakhstan's Ministry of Foreign Affairs issued a statement saying two foreign nationals arrested on Saturday in the Boston area on immigration violations are from Kazakhstan and may have known the two marathon bombing suspects.

The ministry said US authorities came across them while searching for "possible links and contacts" to Dzhokhar Tsarnaev. Their names have not been released.

On Monday, Boston residents observed a moment of silence exactly a week after the first of two bombs went off near the marathon finish line.

A private funeral was also held for Krystle Campbell, a 29-year-old restaurant worker who was among the dead.

Later in the day, a memorial service was held at Boston University for another victim, Lu Lingzi, 23, a graduate student from China.

The youngest person to die was eight-year-old Martin Richard, while police officer Sean Collier was killed during a confrontation with the suspects in the early hours of Friday.