Japanese police arrested four people for allegedly threatening copycat killings online after a man posted similar messages on Web sites before stabbing seven people to death, officials said Tuesday.

"I'm sick of it all. I'm going to do it too," a jobless 29-year-old wrote on the hugely popular 2-Channel Web site. "I'm going to kill 100 people in Ikebukuro," he said, referring to a popular Tokyo shopping district, a Tokyo Metropolitan Police official said on condition of anonymity, citing departmental policy.

The suspect allegedly posted the message a day after the June 8 stabbing attack in another Tokyo district, Akihabara, the official said.

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In the June 8 attack, a man rammed pedestrians with a truck, jumped out and knifed more than a dozen people. Seven people died and another 10 were injured.

Tomohiro Kato, who was arrested on the scene splattered with blood, foretold the attack in hundreds of messages on the Internet. The messages continued until 20 minutes before the killing began — the last one saying, "It's time."

Similar messages have shown up repeatedly in the days since that massacre.

In Hiroshima, police arrested a 19-year-old man Sunday over a message allegedly posted on June 9 saying, "That incident in Akihabara ... I will kill everyone" at a popular local shopping street, police spokesman Hideo Yamamoto said.

In northern Yamagata, a 29-year-old man was arrested Thursday for allegedly posting a message saying he planned to run over people with his truck. At Chubu International Airport in central Japan, a 24-year-old man was detained after allegedly vowing in a message to "stab people" at a nearby train station Monday, officials said.

A junior high school student was also questioned in northern Niigata after allegedly posting a note saying he would burn down a train station on June 30 and then "randomly commit murders."

Police have closely monitored the Internet since the Akihihabara rampage, intensifying attention that began over suicide Internet sites that show viewers how to kill themselves by mixing household chemicals and inhaling the toxic fumes. National Police Agency is also urging knife shops to restrain from selling dagger knives similar to the one used in the Akihabara attack.

"We must crack down on these threats, even though many of them could be just hoaxes," said Hiroshima police official Hideo Yamamoto.