Prosecutors use blood, DNA to show bomber's active role in marathon attack, police killing

Prosecutors are using DNA evidence and blood to try and prove that Boston Marathon bomber Dzhokhar Tsarnaev (joh-HAHR' tsahr-NEYE'-ehv) and his brother both played prominent roles in the 2013 attacks and the killing of a Massachusetts Institute of Technology police officer.

A DNA expert testified Wednesday that Officer Sean Collier's blood appeared on white gloves found in Tsarnaev's car.

Jennifer Montgomery, a DNA analyst with the state police crime lab, said Collier's blood was on a pair of gloves found near the gas pedal and driver's seat of Tsarnaev's Honda Civic. Prosecutors say the brothers fled the scene of Collier's shooting in the Honda.

During cross-examination, Tsarnaev's lawyers established that Tsarnaev didn't have Collier's blood on the sweatshirt he wore that night.

Testimony resumes Thursday.