The Metropolitan Police Department's Public Security Bureau has been put on an increased level of alert against terrorist attacks that include the use of homemade bombs, which can be made from such materials as gas cylinders and readily available chemicals.
There have been a series of incidents around the world involving so-called "lone wolves," or young people who are believed to commit violent acts without belonging to any terrorist groups, while international terrorist groups are providing information on how to produce homemade bombs.
With such unsettling trends in mind, and ahead of the 2020 Tokyo Olympics and Paralympics, the bureau has been accelerating efforts to address the issue, taking measures such as asking relevant industry organisations to report to police when dubious customers visit their shops.
In March, a group related to the international terrorist organisation Al-Qaida provided information on a limited access website how to produce a bomb from a large gas cylinder, along with pictures.
The group has also provided information on how to make a pressure cooker bomb, and other information concerning homemade bombs that can be made from commonplace materials.
In the Boston Marathon bombings in April 2013, the arrested suspect is believed to have referred to the information provided by the group to produce the bombs, prompting public security authorities in many countries to monitor the group's moves.
Gas cylinder bombs, such as those used in terrorist attacks in China's Xinjiang Uygur Autonomous Region in 2008, can pack lethal force.
Strongly concerned that the information on how to make such bombs is available on the Internet, the Public Security Bureau prepared a document asking about 2,000 operators handling high-pressure gas to confirm the identities of purchasers or immediately report to police when suspicious customers visit them.
The bureau also launched serious efforts to address the problem by calling on retail shops or those selling related products on the Internet to cooperate via relevant industry organisations.
Since 2003, the MPD has been asking retail shops and others to offer information about customers who buy 11 specific items, including chemicals, that could be used for making bombs while promoting public and private cooperation in antiterrorist measures. With these latest measures, the MPD enhanced such cooperation.
Meanwhile, the bureau also boosted its cyberpatrol activities.
The bureau has asked Internet-service providers to block websites containing information on how to make bombs, but the sheer vastness of the Internet makes it impossible for every such site to be blocked.
"Shops providing information are vital to helping us nip terrorist attacks in the bud," a senior MPD investigator said.
There were a series of domestic attacks from January to April using gas cartridge bombs at places such as a discount store and a dormitory for Hokkaido police officers in Sapporo.
Amid such incidents, there is increasing concern over lone-wolf terrorist attacks where individuals who do not belong to any groups produce a bomb using commercially available chemicals and other materials.