About 2,000 law officers and their technology swarmed the scene.
Another firefight between the suspect and police erupted.
In the night air, a police helicopter used infrared cameras to see Tsarnaev's movements inside the boat.
"The technology is called 'flir,' forward looking infrared," said former FBI assistant director Tom Fuentes, also a CNN contributor. "The human body is warmer than the air around him, so it stands out."
Knowing the fugitive was alive and moving, police negotiated with Tsarnaev.
Bloodied and injured, he surrendered that night, his capture made possible by police, thermal-imaging technology and a citizen's wisdom to run to the phone.