A 48-year-old man has been sentenced to life in prison for killing a girl at her home in west London.
Katerina had arrived in Britain from Macedonia in 1995
Polish-born Andrezej Kunowski, of Acton, west London, had denied murdering Katerina Koneva, claiming it was a case of mistaken identity.
The 12-year-old was strangled at her home in Hammersmith on 22 May 1997 after she returned from school.
The Old Bailey was told Kunowski was on the run for raping a 10-year-old girl in Poland at the time of the murder.
He previously served 10 years in jail in Poland for 27 serious sex attacks on girls and women from the age of 17.
After the verdict, police said they would be questioning Kunowski about other unsolved cases, including the murder of 13-year-old Milly Dowler.
She vanished on 21 March, 2002, while walking home from Walton-on-Thames railway station in Surrey.
Her body was found seven months later in woodland near Fleet, Hampshire.
Sentencing him, the Common Serjeant of London, Judge Peter Beaumont, said that for Kunowski life would mean life to protect others.
Det Ch Insp David Little said: "He is probably the most dangerous sex offender I have ever come across and certainly the most prolific."
In a statement, Zaklina Koneva, Katerina's mother, said: "The only feeling I have is one of relief. I am relieved that this evil man is no longer free to murder or sexually assault another young girl."
Katerina's father, Trajce Konev, returned home in 1997 and came face to face with an intruder and chased him into the street.
Kunowski was charged with murdering the girl when his DNA, which was taken after he was arrested for raping a student, matched a hair found on Katerina's cardigan.
He also left fingerprints as he leapt out of a first-floor window of the house in Iffley Road.
During the trial, Mr Konev told the court he looked through the keyhole of the barricaded door and saw his daughter's bag in the room.
And when he looked under the door he saw a pair of men's black shoes.
"I started like mad kicking it with all my body. I was trying just to get in, because my brain was saying I knew something was wrong," Mr Konev told the jury.
He said he then ran downstairs and came face to face with the man outside bedroom window.
"I looked at him. He was so cool. It was so strange. Then I noticed a little bit of blood on the side of his face," Mr Konev said.
"I just asked what he was doing in my house. He just looked at me and ran away. I went after him."
He eventually lost the man, the court was told.
When Mr Konev returned to the flat he forced the door open.
Katerina was found lying with a cord tied tightly round her neck, the jury heard. It had been given to her as part of a present after she arrived in Britain from Macedonia in 1995.