The surviving Boston bombing suspect has not yet been interrogated as he recovers in hospital from wounds sustained in a shootout, police said Sunday.
Dzhokhar Tsarnaev, 19, is in "critical but stable condition," Boston Police Department Commissioner Ed Davis told reporters.
He was taken into custody bloodied and seriously wounded Friday after an hours-long manhunt and an earlier shootout that killed his older brother Tamerlan, 26.
"The suspect is not yet able to be interrogated by police," Davis said.
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.
Yet "we don't know if we'll ever be able to question the individual," Boston Mayor Thomas Menino earlier told ABC television's "This Week" program.
Reports said charges against Tsarnaev could come as early as Monday.
Authorities have yet to disclose the exact nature of Tsarnaev's injuries as he receives treatment at Beth Israel Deaconess Medical Center, where some of the 180 people wounded in the blasts are also being treated.
Three people died in the attacks, the worst to take place in the United States since the suicide airliner bombings on September 11, 2001.
The police chief said the FBI may reopen Copley Square, the area where the double explosions took place by the finish line of the Boston Marathon on Monday.
He said federal authorities were trying to track down how and where the two suspects obtained the firearms and explosive devices.
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.