MARTINEZ -- The last known witness for the prosecution in the Richmond High School gang rape dual jury trial testified Monday that DNA from both defendants was recovered from the crime scene.

Defendant Marcelles Peter's DNA was found on the interior of a discarded, broken used condom, according to DNA expert David Stockwell, who retired from Contra Costa County's crime lab in 2011. Defendant Jose Montano's DNA was found on a condom wrapper, Stockwell said.

Peter, 20, and Montano, 22, are each facing life in prison if they are convicted as charged in connection with the Oct. 24, 2009, assault on a then-16-year-old girl in a dark campus courtyard during a homecoming dance. Questions that their attorneys lobbed at Stockwell on Monday focused on the integrity of the evidence collection and testing processes, and possible scenarios as to how their clients' DNA could get on the objects in an effort to raise doubt with jurors.

Ari Morales, 19, and Manuel Ortega, 22, are already serving long prison sentences after taking plea deals in the case; Elvis Torrentes, 25, and John Crane, 46, are awaiting trial.

The lab spent a year testing all the biological evidence against DNA samples that police collected from the victim, the seven men and boys originally charged in the case, and another six men and boys who were identified as witnesses to the assault.

From the victim's body, authorities identified sperm DNA from Crane, Ortega and a male who has never been identified by police, Stockwell said.

Stockwell said he could not say whether Peter's genetic material retrieved from inside a sperm fraction sample on the condom came from Peter's sperm. It is possible, he said, that Peter's non-sperm DNA, which was also found on the condom, crossed over into the sperm fraction.

Crane's sperm was recovered from that same condom, as well as on numerous parts of the victim's body, on stains on other suspects' clothes and on various objects found at the crime scene, according to Stockwell.

Montano's DNA on a condom wrapper was of the non-sperm variety, Stockwell said. Both Peter's and Montano's DNA were also found on an alcoholic beverage can.

Prosecutor John Cope began presenting witnesses June 4, and Stockwell is his last witness unless he chooses to call a rebuttal witness to contradict testimony by any defense witnesses. Contact Malaika Fraley at 925-234-1684. Follow her at Twitter.com/malaikafraley.