Italian investigators say police used cell phone records to track down one of the suspects in the failed suicide bombings in London on 21 July.
Hussain Osman was arrested on Friday in his brother's flat on the outskirts of the Italian capital, Rome.
He was traced using call records from two cell phone numbers, supplied to the Italians by UK police.
Police believe he travelled by train across Europe from London via Paris, Milan and Bologna.
Mr Osman is wanted in the UK over the attack on Shepherd's Bush underground station.
The suspect's constant use of cell phones betrayed his attempt to find refuge. As well as calling his brother in Rome, he talked to his father who lives in Brescia, in northern Italy.
Carlo De Stefano, head of the Italian Anti-Terrorist Force said that on 26 July - the day UK police informed their Italian counterparts that Mr Osman had left Britain - checks were carried out on Italian phone numbers that had, in the past, been in contact with Mr Osman.
Italian authorities checked whether Mr Osman had contact with Ethiopians and Eritreans in Milan and Brescia, where his father lives.
One particular call in Milan was thought to be interesting to police, Mr De Stefano said.
Following information from the Metropolitan Police that a friend of Mr Osman's was thought to be using a phone in Italy, Italian police were able to trace the phone but realised it was Mr Osman himself who was making calls.
But the phone went dead for several hours on 27 July, but when it was switched on it had changed from the UK network to an Italian network.
Although the SIM card had been changed, the handset was the same, allowing police to track it to a specific area in Rome on 28 July.
Police were also monitoring another Ethiopian in Rome - Mr Osman's brother, who ran a phone and internet cafe. Calls were being made to him.
Calls on the phone thought to be Mr Osman's were made in an Ethiopian dialect, which police recorded and sent to the Met police for verification that it was Mr Osman's.
The confirmation meant police were faced with the presence of the alleged attacker, Mr De Stefano said.
A phone centre and internet cafe run by his brother near Rome's Termini railway station was also searched by Italian police.
The brother in Rome and another in Brescia were also arrested.