The official said that a posting on a social media site in the elder brother's name included the comments: "I don't have a single American friend. I don't understand them."
Dzhokar Tsarnaev attended Cambridge Rindge & Latin, a public high school, said Eric Mercado, who graduated a year behind the suspect. Mercado said Tsarnaev had worked at Harvard University as a lifeguard.
"We hung out; we partied; we were good high school friends," Mercado told CNN.
"We're all, like, in shock. We don't really understand. There were no telltale signs of any kind of malicious behavior from Dzhokar. It's all coming as a shock, really."
Mercado said he lived a block away from the suspect and did not know his older brother.
Dzhokar Tsarnaev is currently registered as a student at University of Massachusetts Dartmouth, which ordered its campus evacuated on Friday. The school is located 65 miles south of Cambridge, just west of New Bedford.
Larry Aaronson, Dzhokar Tsarnaev's neighbor and a former teacher at the high school Tsarnaev attended, called him a "wonderful kid."
"He was so grateful to be here, he was compassionate, he was caring, he was jovial," Aaronson told CNN.