BOSTON - Every time the widow of suspected Boston Marathon bomber Tamerlan Tsarnaev leaves her parents' house, federal agents watching the residence follow her in unmarked vehicles.

Federal authorities are placing intense pressure on what they know to be the inner circle of the two bombing suspects, arresting three college friends of surviving brother Dzhokhar Tsarnaev, and keeping Tamerlan's 24-year-old widow, Katherine Russell, in the public eye with their open surveillance and leaks to reporters about investigators' focus on her.

Legal experts say that is part of the quest not just to determine whether Russell and the friends are culpable but also to push for as much information as possible regarding whether the bombing suspects had ties to a terrorism network or accomplices working domestically or abroad. A primary goal is to push Russell and friends to cooperate fully, according to the experts.

David Zlotnick, a professor of law at Roger Williams University in Rhode Island and former federal prosecutor in the District of Columbia, said authorities may be tracking Russell closely because they think she is not being completely honest about all she knows.

"It seems to me they don't believe her yet," he said.

Dzhokhar Tsarnaev is in a prison hospital, facing a potential death sentence if convicted of the terrorism plot that authorities allege the 19-year-old and his 26-year-old brother carried out April 15. Twin pressure-cooker bombs detonated near the race's finish line, leaving three people dead and injuring more than 260. Tamerlan Tsarnaev died in a gunfight with authorities April 19, a day after authorities released photos of the suspects.

Russell has been ensconced at her parents' North Kingstown, R.I., home since then. Much about her remains a mystery, including what she knew or witnessed in the weeks, months, and years before the bombings, and what she saw and did in the days after.

It is unclear when Russell last communicated with her husband, but her lawyer, Amato DeLuca, told the Associated Press last month that the last time she saw him was before she went to work April 18. DeLuca said Tuesday that Russell had met with investigators "for many hours over the past week" and would continue to do so in the coming days.

He previously told the AP that Russell did not suspect her husband of anything before the bombings and that nothing seemed amiss in the days after.

Zlotnick said the fact that charges have been brought against the younger brother's three friends from the University of Massachusetts Dartmouth over allegations that they covered up for him indicate authorities are willing to go after the widow for similar actions. That puts pressure on Russell to cooperate.

Dias Kadyrbayev and Azamat Tazhayakov, students from Kazakhstan, were charged last week with conspiring to obstruct justice by taking a backpack with fireworks and a laptop from Dzhokhar Tsarnaev's dormitory room. Robel Phillipos was charged with lying to investigators about the visit to the room.

On Saturday, Jeff Bauman, 27, who lost both legs in the blasts, was honored at Game 2 of the NHL playoffs in Boston. Bauman, who has family ties to the Philadelphia area, said at the ceremony: "I'm making great progress, and I thank everyone for their thoughts and prayers."