TOKYO - The use of informal language by a university junior so upset Hirotaka Hanamori that he bore a years-long grudge that led to an acid attack at a Tokyo subway station last week.

Mr Hanamori, 25, was nabbed on Aug 28 in Okinawa, four days after the attack at about 9.05pm at the Shirokane-Takanawa station in Tokyo.

Just as the world's attention was on the opening ceremony of the Paralympic Games, Mr Hanamori stalked his 22-year-old target, who remains unnamed, for three subway stations before the attack.

Mr Hanamori, who was on Monday (Aug 30) referred to prosecutors but has not been charged, allegedly flung sulphuric acid at his victim.

The man, who works at a real estate firm in Tokyo, suffered severe injuries to his face and neck as well as cornea damage to both eyes.

Doctors say he will not be blinded, but his injuries will take six months at least to heal.

Another 34-year-old woman who was passing by was also injured, though the police believe that she was not directly involved in the case and was likely at the wrong place at the wrong time.

Violent crime is relatively rare in Japan, though this was the second attack on Tokyo's train network this month.

They came despite a heightened security alert for the Tokyo 2020 Olympic and Paralympic Games.

Earlier this month, Yusuke Tsushima, 36, went on a stabbing spree on board a rapid express Odakyu train near the Seijo-Gakuen station.

Tsushima was spurned in love, and wanted to attack "happy-looking women".

Ten people were injured in that attack, one of them seriously, before Tsushima turned himself in to the police the same night.

Mr Hanamori, however, had no intention of surrendering.

He fled after the attack, first by shinkansen bullet train back to his home in Shizuoka City and then by plane the next day to Okinawa via Nagoya.

He was placed on Japan's nationwide most wanted list and the police slowly pieced together his motives over four days after the attack, which seemed random at first.

Both Mr Hanamori and his victim were schoolmates at the University of the Ryukyus in Okinawa, where Mr Hanamori read agriculture.

They were both part of the same film extracurricular circle.

But Mr Hanamori allegedly yelled at his victim once during a club gathering, for speaking to him using casual, informal language.

The Japanese language adopts honorific speech, with different vocabulary and grammatical forms used for different levels of respect.

A senior, for example, is typically called "senpai" or will have the word "san" attached as a suffix to their last name unless they are on close, first-name terms.

The misuse of a term would usually cause consternation in private, but Mr Hanamori flew into a rage: "How dare you use casual language with someone who is older?"

But the suggestion that a man could get so riled up by such a minor trigger has set Japan's social media abuzz, with many saying that they find the motive incredible.

The attack came after Mr Hanamori once confronted his victim last month, reports said citing police sources, when he accused him of being a disgusting brat with bad manners. It is unknown how the victim reacted at the time.

Still, acquaintances painted Mr Hanamori as a social recluse with few friends. He has said that his love interest was an alien.

He wanted to be a biologist and would go into lengthy conversations with anyone who strikes up a chat about insects and beetles.

Mr Hanamori, who dropped out of the University of the Ryukyus, is enrolled as a student in the agriculture faculty at Shizuoka University.

It remains unclear how he obtained the sulphuric acid.

The son of a wealthy chiropractor father and a mother who works in the medical field now lives alone in a two-storey detached house in Shizuoka, with both his parents having passed away.

He fled to Okinawa to hide, staying with an unwitting friend until he was traced on Saturday.

He tried to give the police the slip by claiming that his name was Sunagawa and had hundreds of thousands of yen in cash on him when nabbed.