Aaron Deveau is standing trial for the death of Donald Bowley Jr., 55, of Danville, N.H., and the severe injuries of Bowley's girlfriend, Luz Selena Roman, The Boston Herald reported.
Prosecutors said the crash on Feb. 20, 2011, occurred because Deveau was texting while driving, causing him to veer into oncoming traffic and strike Bowley's car.
Deveau, then 17, said he wasn't texting before the collision, but rather became nervous when the car in front of him began braking, so he swerved to avoid hitting that car.
Retired Westboro police officer Ronald MacKendrick, a defense expert in reconstructing accidents, said Deveau had an "involuntary reaction" to the car suddenly braking in front of him.
However, the driver of the car in front of Deveau's, Robin Murphy, testified last week that she didn't know Deveau was behind her and had no reason to brake until she heard the crash. She also testified stopped to help, she said she saw a cellphone on the front seat of Deveau's car.
Prosecutors also said last week that Deveau sent or received 193 text messages in the hours and minutes leading up to the accident.
If convicted, Deveau faces up to four years in jail.