Police had no idea whether he had weapons or explosives with him, so they repeatedly told him to stand up and lift his shirt to show he wasn't wearing a device, Deveau said.
An FBI negotiator was on the second floor of the house looking down at the boat, but he couldn't see Tsarnaev under the tarp. A state police helicopter overhead used a heat sensor to determine that Tsarnaev was alive and moving, Deveau said.
Eventually Tsarnaev stood up and lifted his shirt for the officers.
"At that point, once we saw that, we felt comfortable enough to send some officer tactical equipment to grab him and pull him away from the boat," Deveau said. "He needed first aid, so he was transported by ambulance into a Boston hospital."
Investigators are still combing through all the crime scenes, including the boat, so Deveau said he didn't have details yet on what items Tsarnaev may have had with him when he was captured. And they still don't know where the brothers allegedly got the weapons, explosives and know-how.
Police are confident, however, that there are no other suspects, Deveau said.
"From what I know right now, these two acted together and alone," he said. "As far as this little cell and this little group, I think we got our guys."