US intelligence agencies added the mother of the Boston bombing suspects to a government terrorism database 18 months before the bombings, two officials said.

She called it "lies and hypocrisy" and said she has never been linked to crimes or terrorism.

The CIA asked for the older suspect, now dead, and his mother to be added to a terrorist database in the autumn of 2011, after the Russian government contacted the agency with concerns that both had become religious militants, according to officials briefed on the investigation.

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About six months earlier, the FBI investigated Tamerlan Tsarnaev and his mother, Zubeidat Tsarnaeva, also at Russia's request, one of the officials said. The FBI found no ties to terrorism.

The younger suspect, 19-year-old Dzhohkar Tsarnaev, was moved overnight from a hospital to a federal prison medical centre to continue his recovery from a throat wound and other injuries suffered during a getaway attempt. Tamerlan Tsarnaev, 26, died in a shootout with police.

Also, FBI agents picked through a landfill near the campus of the University of Massachusetts Dartmouth, where Dzhohkar Tsarnaev was a student. FBI spokesman Jim Martin would not say what investigators were looking for.

An aerial photo in Friday's Boston Globe showed a line of more than 20 investigators, all dressed in white overalls and yellow boots, picking over the garbage with shovels or rakes.

The revelation that the FBI had also investigated Zubeidat Tsarnaeva and the CIA arranged for her to be added to the terrorism database deepened the mystery around the family. The Tsarnaevs are ethnic Chechens from southern Russia who immigrated to the Boston area in the past 11 years. Tsarnaeva, a naturalised US citizen who has appeared on television interviews since the attacks and reversed her decision to return to the US after the bombings, has said her sons could never have been behind the deadly attacks and believes they were framed.

Dzhohkar Tsarnaev is charged with joining with his older brother in setting off the shrapnel-packed pressure-cooker bombs. He could face the death penalty.

Officials said that before he was advised of his constitutional rights to remain silent or consult a lawyer, Dzhokhar admitted to FBI interrogators that the brothers committed the bombings and that he was recruited by his brother to participate only a week or two before the attacks.

Dzhokhar was taken overnight from Beth Israel Deaconess Medical Centre and transferred to the Federal Medical Centre Devens outside Boston, the US Marshals Service said. The facility at the former Fort Devens Army base treats federal prisoners.

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Previously US officials have said only that the FBI investigated Tamerlan. But in March 2011, the Russians asked the FBI to look into Tamerlan and his mother because of concerns they were religious militants who planned to travel back to Russia, the official said.

The FBI found nothing to link either person to terrorism, and the FBI closed the investigations in June 2011. Then, the Russians in the autumn sent the same warning to the CIA. The CIA asked the US National Counterterrorism Centre to add the mother's and son's names to its huge, classified database of people known to be terrorists and those who are suspected of having terror ties, called the Terrorist Identities Datamart Environment, or TIDE.

Being in that database does not mean the US government has evidence that links someone to terrorism. About a year ago, there were some 745,000 names in the database. Intelligence analysts add names and partial names to TIDE when terror-related intelligence is shared with them.

Tsarnaeva said it would not surprise her if she was listed in a US terror database.

"It's all lies and hypocrisy," she told the AP from Dagestan. "I'm sick and tired of all this nonsense that they make up about me and my children. People know me as a regular person, and I've never been mixed up in any criminal intentions, especially any linked to terrorism."

A search of US criminal records showed only that Tsarnaeva was arrested in June 2012 in Natick, Massachusetts, on a shoplifting charge over the theft of 1,624 dollars (£1,048) worth of women's clothing from a Lord & Taylor department store. She was arrested and charged with larceny and two counts of malicious or wanton property damage. Tamerlan had travelled to Russia in January 2012 and returned in July.

Tsarnaeva accused US law enforcement of killing her elder son.

"They are already talking about that we are terrorists, I am terrorist, they've told that I was doing something terroristic," Tsarnaeva said.

A team of investigators from the US Embassy in Moscow has questioned both parents in Russia this week, spending many hours with the mother in particular over two days.

Some lawmakers in Washington have questioned whether the FBI adequately investigated Tsarnaev and his mother in 2011. Over the course of that year, the FBI reached out to Russia three times for more information, US officials said. The first time was in March 2011, when they received the initial tip from the Russians. The second was in June 2011 when they were preparing to close the investigation. The third time was in the fall of 2011 after the CIA received the same tip from the Russians.

One of the officials said the FBI never found the type of derogatory information on Tsarnaev and his mother that would have elevated their profiles among counterterrorism investigators or would have formally placed them on a terror watch list.

AP