A student accused of changing his grades while working in the admissions office at Diablo Valley College, in California, was acquitted on all charges after his two-week trial ended this month, according to The Advocate, the student newspaper at the college’s sister institution, Contra Costa College.

The jury found that while 15 of his grades had been changed to A’s, there was not sufficient evidence to suggest that the defendant, Erick Martinez, made the changes himself. Mr. Martinez’s lawyers argued that three other student employees, also charged in the case, conspired against him.

Dodie Katague, deputy district attorney for Contra Costa County, said there was little incentive for the other three, who earlier had pleaded guilty and cooperated with prosecutors, to alter their co-worker’s grades. What’s more, Mr. Katague argued, the changes were made using Mr. Martinez’s username and password.

“We believe we had enough evidence to show that Erick Martinez was the only one that benefited from having [15] of his grades changed,” Mr. Katague told The Chronicle. “He committed the perfect crime and he was able to get away with it by blaming the other defendants.”

The investigation began in 2005 after a professor noticed Mr. Martinez’s name kept reappearing on his class roster with an A grade, despite efforts to drop him from the class on two separate occasions. Investigators soon uncovered what they described as a scheme in which student workers in the admissions office received money for changing grades on the college’s computer. Authorities said the students made more than 400 such changes over several years. One instance involved a student who took out more than $4,000 in credit-card advances to pay for 15 grade changes.

Mr. Katague, the prosecutor, called on instructors, friends, and former co-workers to testify that Mr. Martinez was a D student who, on at least one occasion, told another student he knew how to change grades.

The three other student employees received lighter sentences of one year in jail for their cooperation with prosecutors. Julian Revilleza, who pleaded guilty in the scheme last fall and recently finished serving his sentence, also testified against Mr. Martinez.

According to Judy Walters, the college’s president, corrected transcripts were sent to all four-year universities where students had transferred. Diablo Valley handed out its own punishments in accordance with its student code of conduct, and students faced different lengths of suspension depending on the degree of fraud committed, Ms. Walters said. The university found Mr. Martinez guilty.

“We do not believe that this jury verdict means that Mr. Martinez was innocent,” Ms. Walters said in an e-mail message. “It simply means the district attorney was not able to prove, beyond a reasonable doubt, that Mr. Martinez made the unauthorized grade changes.”

The police are still searching for another former student worker, Ron Nixon, who they allege also participated in the grade-changing scheme. —David DeBolt