Investigators who had been waiting to interrogate Dzhokhar Tsarnaev, 19, since his capture on Friday are trying to determine whether his neck wound was self-inflicted in a suicide attempt, USA Today quoted a federal law enforcement official as saying.
A law enforcement official told AFP that counterterrorism agents trained in interrogating "high-value" detainees had been waiting to question Tsarnaev at Beth Israel Deaconess Medical Center, where some of the 180 people wounded in the April 15 twin blasts are also being treated.
Boston Police department commissioner Ed Davis had earlier told reporters that Tsarnaev was in "critical but stable condition," and had not yet been interrogated.
Three people died in the attacks, the worst to take place in the United States since the suicide airliner bombings on September 11, 2001.
In addition to his neck injury, reportedly a gunshot wound through the mouth that exited to the back of his neck, Tsarnaev is said to have been shot in the leg during a shootout with law enforcement the night before his arrest.
A policeman was killed and another was seriously wounded in the shootout.
That altercation killed his older brother and fellow suspect Tamerlan, 26, and triggered an hours-long manhunt for the surviving teenaged suspect that shut down Boston.
The two were ethnic Chechens who had been living in the United States for a decade.
Davis, the police chief, said the FBI may reopen Copley Square, the area where the explosions took place by the finish line of the Boston Marathon.
Speaking on CBS television's Face the Nation, Davis said the Tsarnaev brothers were armed for another attack.
"They had IEDS," Davis said referring to improvised explosives devices.
"They had homemade hand grenades that they were throwing at the officers.
"The scene was loaded with unexploded improvised explosive devices that actually we had to point out to the arriving officers and clear the area," he added, noting that one IED was found in a Mercedes sports utility vehicle the brothers had abandoned.
"This was as dangerous as it gets in urban policing."
He said federal authorities were trying to track down how and where the two suspects obtained the firearms and explosive devices.
Officials have invoked a "public safety" legal exception that will allow them to question Tsarnaev without reading him his so-called Miranda rights to remain silent and to consult a lawyer.
Some Republican lawmakers have called for declaring the teenager an "enemy combatant," which would give him the same status as Guantanamo "war on terror" detainees.
But Senator Lindsey Graham has since nuanced his comments, stressing that Tsarnaev should only be considered an enemy combatant for interrogation purposes, and should receive a civilian trial in federal courts, rather than in a military tribunal like foreign suspects with that designation.
Critics have insisted that because Tsarnaev is a naturalized US citizen and authorities have found no ties between him and terror groups so far, he should be granted a criminal civilian trial.
US lawmakers are also questioning why Tamerlan, who may have been radicalized or even trained in the Caucasus, did not raise more red flags despite being questioned at the request of the Russian government in 2011 and spending six months in the volatile region last year.
"Clearly, something happened in my judgment in that six-month timeframe... I'm very concerned," House Homeland Security Committee Chairman Michael McCaul told CNN's "State of the Union."
Graham said it was a "mistake" to have let the older Tsarnaev out of law enforcement's sight.
"I don't know if our laws are insufficient or the FBI failed, but we're at war with radical Islamists and we need to up our game," he said.
The Tsarnaev family came to the United States from the former Soviet state of Kyrgyzstan around 2002. Dzhokhar became a US citizen in 2012, while his older brother's application was reportedly held up.
Tamerlan began posting militant videos on social media sites in recent years, and traveled to Dagestan, which borders Chechnya, in 2012. Both Russian regions host separatist rebel groups.
The brothers' social media pages appeared to express sympathy with the struggle in Chechnya, which has been ravaged by two wars since 1994 between Russia and increasingly Islamist-leaning separatist rebels.
But a website used by rebels in Dagestan denied any link to the Boston bombings, saying it was "not waging any military activities against the United States."
"We are only fighting Russia, which is not only responsible for the occupation of the Caucasus, but also for monstrous crimes against Muslims," the Kavkazcenter.com website said.