The 15-year-old, identified only as Pierre, has confessed to a two-hour killing spree last year during which he waited calmly for each member of the family to return home before firing on them with his father's shotgun. In between the shootings he sat on the sofa and watched a video of the film Shrek.

The public prosecutor, Josef Schmit, described the tragedy in a small village near Rouen as "an inexplicable and terrifying massacre", adding that he had "never seen the like in a 30-year acquaintance with crime". For all who knew him, Mr Schmit said, Pierre was "a nice boy from a nice family".

Draped in a blanket, the teenager arrived for the closed-door hearings at the Rouen tribunal in a police car shortly before 9am. As a minor he cannot under French law be sentenced to life, but faces a maximum of 20 years in jail.

According to court documents, on Wednesday October 27 2004 Pierre, who had an essay to finish, stayed at home after lunch while his father, a boilermaker, returned to work, his mother Lydie and four-year-old brother Louis went out to shop, and his sister Marion, 11, attended a dance class. French children do not go to school on Wednesdays.

He told police later that it was at that moment that he had "the idea of killing" his parents. He was fed up, he said, with being "badly treated", especially by his mother, who had forbidden him to go out and play football that afternoon because of his essay. Court appointed experts have said the parents were "relatively strict" but reasonable and loving.

Pierre, who was 14 at the time, loaded his father's shotgun and sat down to watch the film. At 3pm his mother and Louis came home. He shot and killed Lydie as she walked past the front door, then took his baby brother upstairs and gave him some paper and crayons so he would keep quiet.

An hour and a half later his sister came home and Pierre shot and seriously injured her, leaving her slumped in the hallway. Hearing Louis howling in fright at the top of the stairs, the teenager took aim and shot him dead too. He then returned to the sofa and continued watching Shrek.

When at 5pm his father, Thierry, returned from work, Pierre got to his feet and fired two rounds into him at point-blank range. He then grabbed a rucksack, left the shotgun in the hallway, went out, locking the front door and throwing the key into the pond, and climbed on his bike. Marion managed to crawl into the ground-floor bathroom and out of a window. She staggered to neighbours, who called the police. Pierre was arrested soon afterwards, about nine miles from his home, while in a public phone box.

Court appointed psychologists said the teenager was plainly suffering from "a significant alteration of his mental faculties" at the time of the murders, but had not "lost all discernment". During a break in proceedings yesterday a prosecution lawyer, Marie-France Magrin, said Pierre was "calm and natural" in court, and "expressing himself clearly".

A lawyer for the family said last night the first day of the two-day trial had been "exceptionally emotional". Arnaud de Saint-Rémy told French radio that "almost everyone in the courtroom" had found the proceedings "very, very difficult, many were close to tears". So far, she said, he had not expressed any regrets.

Mr de Saint-Rémy said Marion, who had been shot in the stomach, was not in court. "She is still suffering intense psychological shock. It would not be possible for her to be here," he said.

A verdict is expected today.