Pakistani authorities say they are holding seven Islamic militants possibly linked to the London bombings. It also appears that one of the London suicide bombers may have tried to enrol at a madrassa in Karachi last year.
The seven were rounded up as the Pakistani investigation in the bombings focused on Lahore, where one bomber spent time at a religious school. Authorities arrested another 52 people suspected of links to the Islamic militants as part of a sweep across Pakistan.
There are also unconfirmed rumours that Omar Saeed Sheikh, who kidnapped the American reporter Daniel Pearl, has been questioned about the bombings at the Pakistani jail where he is on death row.
The trail of three of the London bombers through Karachi is emerging. When Hasib Hussain, the youngest, visited Karachi in June last year, he tried to join a madrassa, intelligence sources told Pakistani reporters. That was five months before Mohammad Sidique Khan and Shahzad Tanweer visited, when Tanweer is believed to have visited at least two madrassas.
Although the name of the Karachi madrassa has not been confirmed, there are reports that vit is a seminary in the Buffer Zone quarter of the city, founded by Mufti Mohammed Jamil. The madrassa's staff deny any connection to Hussain. It belongs to the same school of Islamic thought as Karachi's renowned Binori Town madrassa, which has overt links to the Taliban.
The hotels where the bombers stayed in Karachi have been identified, but one is an area of the city centre that is not considered safe for Westerners. Visitors are advised to keep as low a profile as possible, and avoid certain areas altogether. It was in Karachi in 2002 that Omar Saeed Sheikh lured Daniel Pearl to his death. It is rumoured that Sheikh has been questioned about the London bombings at Hyderabad jail.
When asked about the attacks, Sheikh replied: "The sapling that I planted has grown into a full tree" - or so the story goes. Like the bombers, Sheikh is a Briton of Pakistani origin. He is also said to have been questioned after an assassination attempt on President Pervez Musharraf in 2003.
When Khan and Tanweer arrived in Karachi on a Turkish Airlines flight on 19 November last year, they headed for the Saddar area, where they checked into the Mustafa Hotel, one of many cheap places in the area where you get little more than a bed and some dirty sheets for your money.
Although it is in the heart of Karachi, Saddar is not considered safe for Westerners. During the 1980s, at the height of Karachi's sectarian violence, more bombs went off in Saddar than anywhere else. But for the bombers it was perfectly safe. "It's not your passport they're interested in there, it's the colour of your face," said one local.
Little has emerged about what Khan and Tanweer did in Karachi during the week they stayed here.
Before they arrived, 18-year-old Hasib Hussain had already passed through Karachi. Immigration records show he arrived on a Saudi airlines flight in June last year, but curiously there is no record of his departure from Pakistan.
During his time in Karachi, he is believed to have stayed at the Day's Inn hotel, on Sharea Faisal - an altogether different type of hotel from the Mustafa.
The Day's Inn is a big, seven-floor place, with a gaudy neon sign. The manager on duty denied Hussain had stayed at the hotel, but admitted that police had been to the hotel asking to see the register and asking questions about Hussain.
The daily rate for a single room is about £50, far higher than the usual rates for a similar hotel in Karachi.