The black widow of the web

For a first date, Naoki Yasuda thought it went pretty well. OK, the woman the 46-year-old was looking at was plainer than the flattering photo she posted on an internet dating site, but she seemed kind, attentive and eager to start a serious relationship. The pair chatted for three hours in her modest Tokyo apartment last September, and by the end, 34-year-old Kanae Kijima agreed to move in with him that week at his house in nearby Chiba Prefecture.

"In the morning she made pancakes, in the afternoon pasta, in the evening fried chicken," recalls Mr Yasuda. "The food was delicious, the house was always clean. She made me tea and I didn't even have to ask for it. I thought she'd make a great wife." It wasn't to be. On 21 September, the police called with the astonishing news that Kijima was suspected of drugging and robbing several men. There was much worse to come. After the cops pulled her in for questioning, a stunned Mr Yasuda watched TV reports describe Kijima as a new type of internet serial killer, implicated in the deaths of at least four men. The latest, 41-year-old Yoshiyuki Oide, was found dead the previous month in what police believed was a faked suicide. Kijima, who is awaiting trial, is suspected of spiking a beef stew with a sedative, then poisoning Mr Oide with carbon monoxide fumes in a rented car – but not before she had swindled him out 5 million yen (£37,000). "I refused to believe it at first," says Mr Yasuda. "I trusted her."

Kijima allegedly defrauded dozens of other men by posing as a prospective marriage partner, home-helper or therapist, posting images of her homemade meals on her website to lure the desperate and dateless. Her haul after just a few years of dedicated con work has been estimated at up to 200m yen (£1.5m). Most of her alleged victims were lonely middle-aged men trawling konkatsu (marriage-hunting) websites. The Japanese media quickly dubbed Kijima the Konkatsu Killer.

The looming trial has sent a chill through Japan's thriving online community. "There are a lot of very lonely people out there who rely on the internet to form relationships and [Kijima] skillfully took advantage of that," says Yuko Kawanishi, a Japanese sociologist who argues a "liberalisation" of Japan's dating market has left many middle-aged men feeling vulnerable. "I'm amazed at how desperate men are: they seem to have such a childish view of marriage. The fact they don't have this social status of marriage makes them lonely."

Mr Oide was a typical victim. Shy, dumpy, lonely and middle-aged, he kept a blog where he posted pictures of plastic model tanks. A few days before his death he typed a final entry: "At 41, I've found someone and I'm going to meet her family, so I may not be able to write as much," he announced. "We're talking about buying a house and starting a new life." He added he was off with his mysterious fiancée on a three-day pre-marriage trip. But on 6 August, his corpse was found slumped in the back seat of a rented car in the one of Tokyo's northern suburbs. Beside him lay a charcoal burner, a common tool for suicide in Japan because it allows the victim to slowly and painlessly asphyxiate.

Heavy-set, squat and with a jowly face, what Kijima lacked in looks she appears to have compensated for with cunning. She cultivated an image of herself on her blog – "Kanae's Kitchen" – and on dating websites as a single woman with a taste for expensive clothes and jewellery – and a desire to find Mr. Right. Cropped and manipulated photos made her appear trim and sexy and she completed a course in French cooking, posting photos of her dishes online. "Human relationships are very difficult, especially male-female relations," she cooed in one posting. "My gentleness is mistaken for love or affection and I sometimes don't know how to react to that. "

Her ability to manipulate and trap the unsuspecting outwitted even those who should have known better. Her first alleged victim, Sadao Fukuyama, a 70-year-old manager of a recycling shop, was lured into her life when she posed on a dating site as an orphaned music student. Kijima agreed to meet him once a month and in return, police say, Mr Fukuyama paid her 3.2m yen (£24,000), supposedly to cover her tuition fees. Eventually, he handed over his bank card, which she used to drain his account of 74m yen (£540,000). He was found dead at home on 6 August 2007; His friends remember him as a careful, thrifty man who would not have been easily duped. "She must have been very persuasive," said one neighbour.

Kijima became adept at transforming herself into what her male suitors needed. For one of her alleged victims, 80-year-old invalid pensioner Kenzo Ando, she appears to have made initial contact via a website for home care. Posing as a nurse, police suspect Kijima got her hands on his cash card and withdrew 1.8m yen (£13,000) before drugging him and setting his house in Chiba Prefecture ablaze last May. Mr Ando's body was later found to contain traces of a sedative.

Other men have told police they took up Kijima's offer to "take her to a nice hotel", then woke up in the morning with a suspiciously heavy hangover and the news that they had agreed to pay her large sums of money. These men appear to have got off lightly. Another of Kijima's reported beaux, 53-year-old Takao Terada, was found dead in his Tokyo home last February, again from carbon monoxide poisoning. Mr Yasuda remembers Kijima also told him that she was struggling financially. "She was going to open a cake shop but the backer pulled out and left her in debt," he recalls. "She was about to lose her apartment and we discussed moving in with me. When I asked her how much debt she had she said she'd check it out and let me know. The next day she said she needed 2.4m yen (£18,000). I put the money in her account."

Investigators closed in on Kijima when they suspected foul play in the death of Yoshiyuki Oide. The missing ignition keys from the suicide car initially raised suspicion. Besides, Mr Oide hardly seemed a likely suicide candidate, having happily told the world about his future with his bride-to-be. An autopsy revealed his last meal had been laced with a sedative. His bank account had been emptied of 5m yen (£37,000).

The police finally closed in on Kijima last September after they tracked her down via Mr Oide's email history. She was asked to attend Tokyo's Kawagoe police station and – in typically Japanese fashion – she politely agreed. Held on fraud charges relating to 12 victims, last month Kijima was finally served a fresh arrest warrant on suspicion of murdering Mr Oide. "We are gathering as much evidence as possible before the trial," explains a police spokesman.

The case is expected to take years to run its course. Japanese police – reputed as being among the most meticulous in the world – are taking extreme care in building their case, but leaks indicate investigators found several types of sedative drug and charcoal briquettes of the type used to kill at least one of the victims when they raided Kijima's Ikebukuro flat.

Meanwhile, in a country where women traditionally play a backseat role, the "Black Widow" case has delivered a devastating blow to male confidence. Bloggers say Kijima has ruined their faith in dating and marriage sites, making them fearful of anonymous encounters with potential online partners.

"How many more Kijimas are there out there?" asks one. "It's terrifying." Mr Yasuda agrees: "Most people are good, but all it takes is one person like her to turn everything bad," adding it has certainly made him "more careful".

"I had no idea of the situation I was in," he continues, staring blankly into the distance. "I'm to blame: I was lonely and easy to deceive. But now I just think how glad I am to be alive."

Naoki Yasuda is a pseudonym

Rogues' gallery: Female serial killers

Miyuki Ishikawa

Known as Oni-Sanba (demon midwife), Ishikawa is believed to have murdered as many as 169 infants between 1944 and 1948 at a Japanese maternity hospital, supposedly because she feared poor parents would be unable to care for their babies. She served just four years in prison.

Rosemary West

Along with husband Fred, West was accused of murdering young women at the couple's home in Gloucester, England, known as the "house of horrors". West, convicted of murdering 10 women in 1995, is serving a life term.

Dorothea Puente

In 1988, Puente was sentenced, aged 60, to life in jail for nine murders. Her victims were elderly, mentally ill tenants of her California boarding house. After killing them, she collected their social security cheques, worth $5,000 a month.