TOKYO—To most of Japan, the deadly stabbings in Tokyo were an incomprehensible act of sudden and indiscriminate violence.

But to readers of an obscure Web site called “Extreme Exchange, Revised,” the attack in a popular shopping district that left seven dead Sunday was foretold.

The suspect sent messages saying he wanted to kill somebody, anybody, and apparently hoped to be stopped. But his words were lost in a million postings.

Police refused to comment on how, or when, they learned of Tomohiro Kato’s postings.

But Japanese news reports Thursday said the suspect told prosecutors under questioning that he deliberately hinted at his bloody plans on the Internet in the hope someone would see them and stop him.

Within hours of his arrest, news of Kato’s Internet activity began to surface and access to the Extreme Exchange site—the bulletin board to which Kato sent postings from his cell phone—was quickly restricted by the site operator.

The Associated Press obtained access to a mirror site—a copy of the original site captured before it was shut down and posted on another server. In it, Kato’s postings offer a wealth of insight into his pre-attack mind-set.

They were generally listed only by number, with no name or even nickname attached.

As early as two months ago, Kato laid out his basic plan of attack, though he added at the time that he wouldn’t carry it out.

Then, as though writing a live feed, on the day of the killings he posted some 30 times, saying he was on his way to the Akihabara area, that he intended to kill and, finally, just 20 minutes before the rampage began, that “it’s time.”

Kato, a 25-year-old factory worker, is accused of ramming a rented 2-ton truck into a crowd of pedestrians at the popular shopping area and then racing around wildly with a military-style dagger and stabbing as many people he could before police finally overpowered him on a side street.

Kato was arrested at the scene with his bloodstained weapon, and police say he has confessed. Under the Japanese legal system, prosecutors have 20 days to formally charge him.