(CNN) -- Evidence uncovered so far supports the theory that the two brothers suspected in the Boston Marathon bombings had no help, authorities said Saturday.
"From what I know right now, these two acted together and alone," Watertown Police Chief Edward Deveau told CNN. "I think we have to be ever vigilant, and we're learning as we go along, but as far as this little cell -- this little group -- I think we got our guys."
Deveau said investigators were continuing to look into where Tamerlan Tsarnaev, 26. and Dzhokar Tsarnaev, 19, may have gotten the resources to amass the cache of weapons used in the bombings and afterward. "We have to figure that out," he said. "There's a lot more work to go."
But, asked whether Boston residents can rest easy, Deveau said, "We got our two guys."
Boston police shared their Watertown colleague's view. "The hunt is over," the department said in a tweet. "The search is done. The terror is over. And justice has won."
But the suspects' father, Anzor Tsarnaev, said his sons were not the bombers.
"My kids never did anything -- that's it," he told CNN from the Russian republic of Dagestan, where he lives. "Never, ever."
President Barack Obama said the question remained open. "Why did these young men who grew up and studied here as part of our communities and our country resort to such violence? How did they plan and carry out these attacks? And did they receive any help? The families of those killed so senselessly deserve answers," he said Friday.
The question of whether others may have played a role in the blasts could affect the decision by federal authorities not to read Miranda rights to Dzhokar Tsarnaev, who was in serious condition Saturday in a Boston hospital.
The government is invoking the public safety exception, a designation that allows investigators to question Tsarnaev without reading him his Miranda rights, a Justice Department official told CNN on condition of anonymity.
In ordinary cases, suspects are told by police they have the right to remain silent and the right to a lawyer.
But this is no ordinary case, according to U.S. Sens. John McCain of Arizona and Lindsey Graham of South Carolina.
They urged that Tsarnaev be held as an "enemy combatant."
"Now that the suspect is in custody, the last thing we should want is for him to remain silent. It is absolutely vital the suspect be questioned for intelligence gathering purposes," the senators said in a statement. "Under the law of war we can hold this suspect as a potential enemy combatant not entitled to Miranda warnings or the appointment of counsel."
Alan Dershowitz, a prominent defense attorney and Harvard law professor, scoffed at the statement.
"There's no way an American citizen committing a domestic crime in the city of Boston could be tried as an enemy combatant," he told CNN's Piers Morgan. "That shows absolute ignorance of the law."
Dershowitz cited statements made by Boston police as appearing to contradict the government's rationale for invoking the public safety exception.
"The police have said there's no public safety issue. It's solved; it's over," Dershowitz said. "There are no further threats. But the FBI is saying there's enough further threats to justify an exception."
But former New York Mayor Rudy Giuliani said state officials may not be privy to information regarding possible international threats that federal authorities may know about.
"You would have to know the internals of what they have before you can assess whether there is a sensible invocation or not," Giuliani said.
Tamerlan Tsarnaev appears to have become increasingly religious in the last three or four years, according to an analysis of his social media accounts and accounts from family members. But there was no immediate evidence of active association with international jihadist groups.
The only video Tsarnaev posted with a strongly jihadist theme appears to have been reposted on his YouTube page four months ago, about the "emergence of the black flags from the promised land of Khorasan." Khorasan is a reference by jihadis to parts of Afghanistan.
And the devices used in the attacks were rudimentary and could have been built by downloading instructions off the Internet, authorities said.
Still, the investigation is continuing. Whether Tamerlan Tsarnaev's return to Russia in the first half of 2012 changed his outlook is likely to be one of many avenues of inquiry. Before he left, the FBI had already -- at the request of an unidentified foreign government -- investigated possible links with militant groups, but discovered nothing suspicious.
A senior U.S. official told CNN it was Russia in 2011 that asked the FBI to look at Tamerlan Tsarnaev's activities.
The request was based on information that Tamerlan Tsarnaev was a follower of radical Islam, the FBI said.
"I think unless we see some horrible dropping of the ball, I don't think this is an intelligence failure," said former CIA operative Robert Baer. "In retrospect, it might look like one, but I don't think it is."
CNN's Barbara Starr, Tim Lister and CNN Terrorism Analyst Paul Cruickshank contributed to this report.