Left, Dzhokhar A. Tsarnaev, suspected of carrying out the bombing...

Left, Dzhokhar A. Tsarnaev, suspected of carrying out the bombing attacks at the Boston Marathon, who was eventually caught hiding in a boat, right, in a backyard in Watertown, Mass. (April 21, 2013) Credit: FBI, Getty Images

BOSTON -- The suspected Boston Marathon bomber has begun responding to investigators in writing, a source familiar with the case said Sunday evening.

Dzhokhar Tsarnaev, 19, was in serious but stable condition at a Boston hospital, according to the FBI. He had been unable to speak due to a neck and throat injury, the source said. He was also intubated and breathing with the help of a respirator and earlier, officials had said he was not in any condition to be questioned.

It was not clear, however, whether what Tsarnaev was writing was of any value.

Authorities believe Tsarnaev may have tried to shoot himself before he was taken into custody Friday night because of the trajectory and location of the bullet wound in his neck, a source familiar with the investigation said Sunday.

The shot was fired at close range, the source said, suggesting the wound was self-inflicted. He was found hidden in a boat in a backyard in Watertown, Mass., bloody and injured. He was able to step out of the boat before being taken into custody after a brief exchange of gunfire with police and then negotiation with FBI agents, said Boston Police Commissioner Edward Davis.

As he lay on a stretcher in the back of an ambulance on the way to hospital, the suspect was mouthing curse words, either to himself or to the armed authorities standing watch in the ambulance as paramedics treated him, the source said.

Prosecutors in the case said Tsarnaev was not charged Sunday.

Asked by reporters whether charges would be filed against Tsarnaev Sunday, authorities released a statement saying, "There will be no press conference or further statement from the U.S. Attorney's Office this evening regarding the Boston Marathon attacks."

Boston Mayor Thomas Menino said at the news conference that authorities are now convinced Tsarnaev and his brother, Tamerlan, who was killed in a shootout with police Friday morning, acted on their own in the bombing.

Davis said investigators also have found a circuit board with a wire slotted through it at the bombing scene on Boylston Street and the area had been swept clean of explosives. Menino said that area is still under the control of the FBI, but will be released to the city soon and a five-phase restoration is planned.

A source involved in the investigation said late Sunday the motorist carjacked by the two brothers as their rampage began last week said the young men stated they planned to drive to New York. The source called such a trip "extremely unlikely" given the extent of the manhunt for the suspects. That assessment was proved correct by the events that followed Friday. However, that tip prompted authorities in Connecticut to be on the lookout for the brothers.

Hundreds of FBI agents and other investigators have interviewed more than 100 people who knew the brothers, the source said. They are looking for any possible links to foreign extremists and seeking to understand how the plot developed and if anyone helped them, the source said.

He confirmed that Tamerlan Tsarnaev had video of a known jihadist on his YouTube channel, footage the source had viewed and characterized as "extremely inflammatory."

Menino had said earlier it has become clear that the older brother, Tamerlan, was the more influential, describing him as the "leader" and Dzhokhar as a "follower."

"His [older] brother read those magazines that are published on how to create bombs," Menino said.

Investigators are also tracking the brothers' online activities, phone calls and text messages, examining hundreds of pieces of evidence -- including bomb components, unexploded devices as well as the brothers' vehicles, guns, and financial records seized from various sites across Boston and surrounding suburbs, authorities said.

The blasts set off near the finish line of the marathon killed three people and injured more than 180.

Sunday morning, Davis said on the CBS News show, "Face the Nation" that the crime scene from the early Friday morning firefight in Watertown, Mass., that killed Tamerlan Tsarnaev was "littered" with unexploded improvised explosive devices.

Davis said it's his belief that the brothers were "going to attack other individuals" -- and Davis said that's based on the evidence at the scene and the firepower that the brothers had.

Two men with connections to the brothers were taken into custody Saturday in New Bedford, Mass., and eventually arrested and detained on immigration violations by Immigration and Customs Enforcement, that agency announced.

The source said those arrests were made in an abundance of caution because the men were friends and former roommates of Dzhokhar Tsarnaev and had regular contact with him. Officials want to question them at greater length about whether they knew anything about the brothers' plans or provided any help to them.

Sunday morning, Boston Cardinal Sean O'Malley implored hundreds of parishioners at a Mass at the Cathedral of the Holy Cross to forgive the suspected Boston Marathon bombers and seize on the goodwill born of the tragedy.

"We must be people of reconciliation -- not revenge," he told about 800 parishioners during a service that honored four killed in last week's marathon bombings and their aftermath. Candles lighted an altar with photos of the dead victims, including the three killed Monday in the bombing, and a police officer killed Friday in a shootout.

Speaking to reporters after the Mass, O'Malley reiterated the church's opposition to the death penalty. "Forgiveness does not mean that we do not realize the heinousness of the crime. But in our hearts, when we are unable to forgive, we make ourselves a victim of our hatred. Forgiveness is part of our obligation of disciples of the Lord."

Also Sunday, Massachusetts Gov. Deval Patrick said on NBC's "Meet the Press" that video footage clearly shows Dzhokhar Tsarnaev placing a backpack near the marathon finish line.

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

Patrick said it's hard to imagine why someone would deliberately harm "innocent men, women and children in the way that these two fellows did."

The FBI had disclosed this week that it had questioned Tamerlan Tsarnaev in 2011 regarding possible terrorist connections, but found nothing and took no action against him.

Sen. Charles Schumer (D-N.Y.) said Sunday he understood that the FBI didn't find any troubling evidence during its initial contact with the suspect in 2011 -- prompted by concerns passed along by Russian authorities -- but he noted that was before Tamerlan Tsarnaev made a suspicious overseas trip.

"Then he went to Russia and he came back and almost immediately put very radical things on his website," Schumer said of the elder Boston bombing suspect. "Why didn't the FBI, having known that he was someone a foreign government at least thought might be dangerous, go interview him again?"

Schumer, speaking during an unrelated news conference, said that the federal death penalty law he helped co-author in the 1990s would be "appropriate" in the case of surviving suspect Dzhokhar Tsarnaev.

On Saturday, Rep. Peter King (R-Seaford) had also questioned the FBI's actions.

Meanwhile, rebels in Russia's North Caucasus region denied any link to the April 15 attack on the Boston Marathon in a statement posted on the vdagestan.com website.

"The Caucasian Mujahedeen are not fighting with the United States of America," according to the statement, which was attributed to unidentified Mujahedeen commanders in Dagestan. "We are at war with Russia, which is responsible not only for the occupation of the Caucasus, but also for heinous crimes against Muslims."

An FBI-led probe is looking into a six-month trip suspect Tamerlan Tsarnaev, an immigrant of Chechen descent, took around two years ago to Chechnya and neighboring Dagestan, both regions of Russia that have been embroiled in Islamist separatist movements.

In Medford, Mass., relatives and friends were expected to attend a viewing Sunday for marathon bombing victim Krystle Campbell, 29, at the Dello Russo Funeral Home, 306 Main St. from 3 to 7 p.m. On Monday at 10 a.m., there will be a procession from the funeral home to St. Joseph Church for a Mass of Christian Burial at 11 a.m. Services will conclude with burial at Oak Grove Cemetery.

The Associated Press reported that doctors said the Boston transit police officer wounded in a shootout with the bombing suspects had lost nearly all his blood and his heart had stopped from a single gunshot wound that severed three major blood vessels in his right thigh.

Surgeons at Mount Auburn Hospital in Cambridge say Richard Donohue, 33, is in stable but critical condition. He is sedated and on a breathing machine but opened his eyes, moved his hands and feet, and squeezed his wife's hand Sunday.

Emergency workers started CPR on the scene to restart his heart. Doctors say he is expected to make a full recovery and that nerves and muscles in his leg are intact.

Transit officials say Donohue had gotten out of his cruiser and was shooting at the suspects when he was hit late Thursday night in a gun battle in Cambridge.

"He went in there and engaged people who were shooting at his fellow officers," Donohue's brother, Edward, said at a news conference Sunday at the hospital. "I cannot describe the pride I have," said the younger Donohue, who is a patrolman for the Winchester Police Department.

The fellow officers included a friend, MIT police officer Sean Collier, who authorities say was shot to death at the beginning of the brothers' rampage earlier in the night.

Richard Donohue has been a transit officer for three years, said Paul MacMillan, chief of the Massachusetts Bay Transportation Authority.

The University of Massachusetts-Dartmouth, where Dzhokhar Tsarnaev was a student, reopened Sunday at noon, university officials announced on the school's website.

With Nicole Fuller,

Paul LaRocco and

The Associated Press

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