When journalist Ruth Lumley replied to graffiti on a train seeking young girls for sex, she thought the scrawled message was probably a sick joke.
Ruth Lumley told the paedophiles she was an 11-year-old girl
But when she received a series of sexually explicit messages in return, Miss Lumley, who had pretended to be aged 11, knew she had to tell police.
Her actions led to a police operation which saw four paedophiles given jail terms ranging from eight years to life.
The judge at Hove Crown Court branded the four men "morally abhorrent".
Miss Lumley, 26, was on a train home to Brighton from Chichester in West Sussex, where she worked as a reporter on the town's weekly newspaper the Chichester Observer, when she noticed the graffiti on the train toilet door.
It read: "Girls 8-13 wanted 4 sex. Girls only text [mobile phone number]. Will pay."
Miss Lumley, who now works for the Argus in Brighton, said after the men were jailed: "It's not the type of message you normally see on the back of toilet doors.
"I phoned the number but didn't get a reply so I just went home and forgot about it.
"About half an hour later I got a message asking me whether I was male or female, how old I was and where I had seen the message.
"I texted back and said I was an 11-year-old girl and that I saw it on the train.
"I had never seen graffiti like this before and at first I thought it was a sick joke.
"After I sent back the reply, I got sent about five or six text messages which got more and more sexually explicit.
"They were really disgusting. It was at that point I knew I had to phone the police."
Officers took Miss Lumley's mobile phone for analysis over a period of five days during which time more sexually explicit messages were sent.
Three of the four men were given life sentences
A British Transport Police officer pretended to be a 12-year-old girl and arranged to meet a man outside a fast food restaurant in Brighton.
The man, Ian Jones, was arrested and when police searched his home they found evidence which led them to the three other men jailed on Tuesday.
They were ringleader Trevor Haddock, 53, from Worcester, who was given six life sentences; Derek Moody, from Gateshead, who was also jailed for life; and John Farmer, 68, from Pevensey in East Sussex, who was jailed for eight years.
Judge Anthony Niblett praised Miss Lumley for her "clear-sightedness" and awarded her £250 to be given to her by the High Sheriff of East Sussex.
Det Con Andy Jackman of British Transport Police, said: "It goes to show that no matter how innocuous something might seem, like a bit of graffiti, it is important to bring your concerns to the attention of the authorities, because in a case like this we've uncovered and put to an end a very serious crime."