(CNN) -- The surviving suspect in the Boston Marathon bombings has indicated that his older brother, not any international terrorist group, masterminded the deadly attack, a U.S. government source said.
Dzhokhar Tsarnaev, who has been hospitalized with a tube down his throat since Friday, communicated with investigators by writing and nodding.
The preliminary interviews with Tsarnaev suggest the two brothers fit the classification of self-radicalized jihadists, the government source said.
Tsarnaev has conveyed to investigators that Tamerlan's motivation stemmed from jihadist thought and the idea that Islam is under attack, and jihadists need to fight back, the source said Monday.
The government source cautioned that the interviews were preliminary, and that Tsarnaev's account needs to be checked out and followed up on by investigators.
Photo released by FBI
A federal law enforcement official told CNN that while investigators have seen no indication the suspects collaborated with others, they still have plenty of work to do before they can confidently say no one else was involved in the April 15 attack that killed three people and wounded more than 260 others. That official would not comment on any motive or specifics on what Dzhokhar Tsarnaev has communicated to officials.
Clues to Tamerlan's trip to Russia
A bedside court appearance
The younger Tsarnaev has been charged with using and conspiring to use a weapon of mass destruction resulting in death, as well as one count of malicious destruction of property by means of an explosive device resulting in death.
The 19-year-old was shot in the head, neck, legs and one hand, according to an FBI affidavit supporting the charges. It's not clear whether he was wounded during his capture Friday night or in an earlier shootout with police that left his 26-year-old brother dead.
Though he had been heavily sedated and on a ventilator at Beth Israel Deaconess Medical Center, Tsarnaev was "alert, mentally competent and lucid" during the brief initial court appearance at his bedside Monday, U.S. Magistrate Judge Marianne Bowler found.
Read a transcript of the hearing
During the hearing, Tsarnaev communicated mostly by nodding his head, though he once answered "no" when Bowler asked him if he could afford a lawyer, according to a transcript of the proceeding. A public defender was appointed to represent him.
Investigators have been asking Tsarnaev whether there are more bombs, explosives caches or weapons beyond those already found by police, and if anyone else was involved in the attacks, a source with direct knowledge of the investigation told CNN. Investigators are going into Tsarnaev's room every few hours to ask questions in the presence of doctors, the source said.
Federal agents at first questioned Tsarnaev without reading him his Miranda rights, under an exception to the rule invoked when authorities believe there is an imminent public safety threat, a Justice Department official said over the weekend. But by the time of the hospital room proceeding, government sources said he had been read his rights, and Bowler reviewed those with him again Monday.
Bowler scheduled a probable cause hearing May 30.
Read the charges
Did the brothers have more plans?
Newly released surveillance images show a stone-faced Dzhokhar Tsarnaev visiting an ATM in the hours before a police chase and chaotic shootout in which more than 200 rounds were fired.
Tamerlan Tsarnaev died at a hospital after that shootout with police early Friday.
A bloody Dzhokhar Tsarnaev was found Friday night, hiding in a boat in the backyard of a house in the Boston suburb of Watertown.
Boston Police Commissioner Ed Davis said the brothers, armed with handguns and explosives, apparently were planning another attack before the shootout derailed their efforts.
"I believe that the only reason that someone would have those in their possession was to further attack people and cause more death and destruction," Davis told CNN on Monday.
Dead suspect's wife 'very distraught'
Tracing Tamerlan Tsarnaev's background
While Tamerlan Tsarnaev apparently became increasingly radical in the past three or four years, there was no evidence Monday that he had any active association with international jihadist groups.
The Tsarnaev family is from the Russian republic of Chechnya and fled the brutal wars there in the 1990s. The two brothers were born in Kyrgyzstan; Dzhokhar became a U.S. citizen in 2012, while Tamerlan was a legal U.S. resident.
An FBI official said agents interviewed Tamerlan Tsarnaev in 2011 at the request of the Russian government. The FBI said Russia claimed that he was a follower of radical Islam and that he had changed drastically since 2010.
But the Russian government's request was vague, a U.S. official and a law enforcement source said Sunday. The lack of specifics limited how much the FBI was able to investigate Tamerlan, the law enforcement official said.
Dead Boston bomb suspect posted video of jihadist, analysis show
Suspects' mother reacts
The suspects' mother, Zubeidat Tsarnaev, said Tuesday that she believed her sons had been framed.
Speaking from her home in Dagestan -- a Russian republic on the Caspian Sea -- Tsarnaev said she didn't believe her older son was dead until she saw images of his body Monday night on the Internet.
Tsarnaev said she believes a conspiracy is at work against her sons, and she doesn't believe evidence put forward by U.S. investigators in alleging their involvement in the bomb plot.
She thinks he died because he was a Muslim and charged that authorities silenced her younger son to prevent him from defending himself.
She said family members have arranged for Tamerlan Tsarnaev to be buried at a mosque in Cambridge, Massachusetts, Tuesday or Wednesday.
University police Officer Sean Collier, allegedly slain by the Tsarnaev brothers Thursday night, will be buried Tuesday in a private funeral in Stoneham, Massachusetts, CNN affiliate WHDH reported.
Collier was found dead in his patrol car on the Massachusetts Institute of Technology campus Thursday night. Authorities believe the brothers shot him to death before stealing a car and leading police on a wild chase in which they fired handguns and hurled explosives at pursuing officers.
A public memorial service is scheduled for Wednesday on MIT's campus, WHDH reported.
Area near blasts to reopen Tuesday
Eight days after the dual bombing turned a stretch of Boylston Street into a gruesome scene of carnage, Boston is now ready to open the area to business owners and residents.
While the site will remain closed to the general public, authorities will escort business owners and residents to the area over several hours Tuesday, the city's website states.
When Bostonians eventually return to Boylston Street en masse, the sight will send a strong message about the city's strength, the police chief said.
"They will be walking up and down this street, and the terrorists will understand that they can not keep us down."
--Jake Tapper and Matt Smith, CNN. CNN's Holly Yan, Michael Pearson, Carol Cratty, Tina Burnside, Fran Townsend, Deborah Feyerick, Jill Dougherty, Nick Paton Walsh, Pamela Brown, Julian Cummings, Barbara Starr and Susan Candiotti contributed to this report.