Authorities are still investigating whether Tsarnaev was armed and whether a shootout occurred at the boat, Evans told reporters.
"Clearly from everybody's vantage point, we could see whoever was in that boat was poking at that top," he said.
It wasn't clear at the time, he said, whether the suspect had a handgun or was trying to poke a hole in the boat's cover. After a standoff, hostage negotiators persuaded him to surrender, Evans said.
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. His brother was a legal U.S. resident.
Although Tamerlan Tsarnaev had become increasingly radical in the past three or four years, there was no evidence Tuesday of any accomplices or connections to extremists, a U.S. official said.
But Dzhokhar Tsarnaev conveyed to authorities that his brother believed Islam is under attack and its adherents must fight back, a U.S. government source said Monday.
Communicating with investigators by writing and nodding, Dzhokhar Tsarnaev has indicated that his older brother masterminded the bombings, the source said.
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 U.S. official said Tuesday that investigators have determined that the pressure cookers used in the attacks were purchased at a Macy's department store.
Suspects' family members react
In a statement issued through their lawyers Tuesday, the suspects' sisters -- Ailina and Bella Tsarnaev -- said they were saddened "to see so many innocent people hurt after such a callous act."
"As a family, we are absolutely devastated by the sense of loss and sorrow this has caused," they said. "We don't have any answers but we look forward to a thorough investigation and hope to learn more."
And Tamerlan Tsarnaev's wife, Katherine Russell, issued a statement through her attorney's office saying she is "doing everything she can to assist with the investigation" and said she and her family are shocked and distraught.
"The reports of involvement by her husband and brother-in-law came as an absolute shock to them all," the statement said.
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 thinks her older son 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.
An official with the Islamic Society of Boston said it has not received any funeral requests from the family. A spokesman for the chief medical examiner's office also said officials there have heard nothing about plans by the slain suspect's family to claim his body.