SWAMPSCOTT — He was a husband, a father, an elementary school reading teacher and, for a time, a Little League coach.

But Stephen Surette had another life, prosecutors say, a secret obsession with pornography of various types, including illicit child pornography.

Surette, 53, of Swampscott, admits he liked to drink and download adult pornography on weekends but says the sexually explicit images of children that investigators found on his computer's hard drive were unintentionally downloaded through a file-sharing program called Kazaa.

Surette will be calling an expert witness who is expected to testify that the same thing could happen to anyone using peer-to-peer file-sharing programs.

Surette's trial on charges of possessing and disseminating child pornography got underway yesterday in Lawrence Superior Court, where a jury of nine women and five men will hear from two computer experts who have come to dramatically different conclusions.

On one side is Richard Falanga, who is the forensics expert for the Essex County district attorney. He will testify about evidence that Surette not only had the images on his computer and on a CD, but had deleted other images and even began using his son's log-in to surf the Web, casting doubt on Surette's claim that the downloads were accidental.

On the other side is Thomas Workman, who was hired by defense lawyer David Twohig. Workman is expected to testify that the images were among batches of what Surette thought was adult pornography.

Twohig is also hoping to bolster his argument by calling as a witness a juvenile justice coordinator for the district attorney's office who recently conducted an information session for parents on the risks of file-sharing software — and offered similar conclusions.

"These programs are a minefield for everyone who's on there," Twohig told jurors in his opening statement.

Prosecutor Jana DiNatale said Surette came to the attention of the Department of Homeland Security in the course of an investigation into German child porn. His e-mail address was found on a computer. The information was passed along until it reached Swampscott police in the fall of 2005.

When police showed up at his door at 32 Allen Road on the morning of May 16, 2006, Surette was getting ready to go to work at Malden's Forestdale School, where he taught fifth- and sixth-graders.

As Surette answered questions from investigators in a sun porch off the kitchen, he kept his voice low, as his wife looked through the closed French doors and his teenage children got ready for school.

"He really wants to keep a secret," DiNatale told jurors.

Surette would admit that he had "granny porn," explicit images of women over 60; "slap happy" videos; and, perhaps, he told investigators, a couple of child porn images.

State police Sgt. Thomas Neff and Swampscott Detective Ted Delano pressed him, pointing out that they were about to seize his computer with a warrant.

"It's going to be a couple, a couple of things I got from Kazaa," Surette told the investigators in a tape-recorded conversation. "I just look at stuff, and I delete it," he told the officers.

A forensic examination would reveal more than "a couple" of images; police found 15, including three that were e-mailed to Surette's America Online account and some that had been saved on a CD.

"Those files don't jump on their own from the computer to a CD," DiNatale told jurors.

Further, there was evidence that Surette had used specific search terms on his computer.

And how could Surette not know a file was child pornography with a title like "Web Cam 10 year young girl," the prosecutor asked.

Some of the files were in a folder in the Kazaa program that allowed other users to access Surette's computer and download the images, leading to the dissemination charge.

And, DiNatale argued, Surette's statements were telling. A "nervous" Surette asked the officers, "Am I in big trouble?" He went on to say, "I never touched anyone or hurt anyone."

Since his arrest nearly three years ago, Surette's appearance has changed dramatically. His lawyer said he is suffering from liver failure and terminal colon cancer and has lost 70 pounds.

But jurors won't be told about his illnesses, after Judge Richard Welch agreed with DiNatale that the information could be considered prejudicial. Surette may be allowed to tell jurors he is taking medication when he takes the stand in his own defense but won't be allowed to disclose his illness.

Welch also rejected a request by Twohig to black out parts of the graphic images being shown to jurors during the trial, saying they could prejudice the jury against his client.

As the images were being shown to jurors yesterday, Welch urged jurors to put aside their emotional reaction and examine them in a calm and rational manner.

The trial is expected to continue through Tuesday.