Five days of terror in Boston

After five days of terror, a clearer picture has emerged of what happened after the Boston Marathon bombing until the death of one suspect and the arrest of another.

With one suspect dead and the other captured and lying grievously wounded in a hospital, the investigation into the Boston Marathon bombings turned to questions about the motives, and to the significance of an overseas trip one of them took last year.

Federal investigators are hurrying to review a visit that one of the suspected bombers made to Chechnya and Dagestan, predominantly Muslim republics in the north Caucasus region of Russia. Both have active militant separatist movements.

There are concerns in Congress about the FBI's handling of a request from Russia before the trip to examine the man's possible links to extremist groups in the region.

An aerial infrared image shows the outline of Dzhokhar Tsarnaev in a boat during the manhunt in Watertown.

Heat seeking: A thermal image of Dzhokhar Tsarnaev in a boat. Photo: Reuters

Tamerlan Tsarnaev, who died early on Friday after a shootout with police in Watertown, Massachusetts, spent six months last year in Dagestan.

His father Anzor said his son had returned to renew his passport, but his stay was prolonged and, analysts said, might have marked a crucial step in his path towards his alleged involvement in the bombing of the Boston Marathon.

Former senior FBI and counterterrorism official Kevin Brock said: ''It's a key thread for investigators and the intelligence community to pull on.''

The investigators began scrutinising the events in the months and years before the fatal attack, as Boston began to feel like itself for the first time in nearly a week.

That week began last Monday with the bombing near the finish line of the marathon, which killed three people and wounded scores. The tense days that followed culminated in Friday's lockdown of the entire region as police searched for Mr Tsarnaev's younger brother from suburban backyards to an Amtrak train bound for New York City.

Dzhokhar Tsarnaev was taken into custody on Friday night after being discovered hiding in a boat stored in a backyard in Watertown and is still too injured to speak.

The motivations of the brothers are as yet publicly unknown. Of Chechen heritage, they lived in the US for years, friends and relatives said, and no direct ties had been established publicly with Chechen terrorist or separatist groups.

The significance of the trip was magnified late on Friday when the FBI revealed that in 2011 ''a foreign government'' - now acknowledged by officials to be Russia - asked for information about Tamerlan, ''based on information that he was a follower of radical Islam and a strong believer, and that he had changed drastically since 2010 as he prepared to leave the United States for travel to the country's region to join unspecified underground groups''.

In a few months, starting last August, the YouTube account in the name of Tamerlan Tsarnaev took on an increasingly puritanical religious tone. It moved from secular militancy to Islamist certainty.

Some of the content Tamerlan viewed included videos of speeches by controversial Sydney Muslim Sheikh Feiz Mohammed.

NSW Police Deputy Commissioner Catherine Burns on Sunday said her unit had been contacted by him after the contents of the YouTube page were revealed.

''We don't hold a concern at this particular point in time,'' she said.

''Sheikh Feiz Mohammed was very clear in condemning the attacks in Boston and he also did say that he does not know the people involved, nor has he had any contact with them.''

with Washington Post, AAP