MEXICO CITY — Four months after the abduction of 43 rural college students shook the nation and set off a political crisis, Mexico’s attorney general on Tuesday officially declared the students dead, saying confessions and forensic evidence supported the theory that their bodies were incinerated near a garbage dump.

The attorney general, Jesús Murillo Karam, delivered a detailed account of the case that did not divert much from what was previously known. Yet he went beyond hints that the students had been killed to declare that after an “exhaustive, serious” investigation, “the evidence allows us to determine that the students were kidnapped, killed, burned and thrown into the river.”

Mr. Murillo Karam, in what appeared to be an effort to convince an increasingly skeptical public that investigators had solved the crime, showed photographs of charred remains, snippets of videotaped confessions and the crime scene. He also disclosed that nearly 100 people had been arrested, 39 confessions obtained and thousands of fragments of human remains recovered.

Over somber music, he played a short video account of the night of the crime, based on what investigators had learned.