Britain's anti-terrorist police chief Peter Clarke, speaks at a press conference at Scotland Yard, 29 Jun 2007

Britain's anti-terror chief says the probe of failed terrorist attacks in London and Glasgow is "extremely fast moving," and says links between the two events are becoming "clearer and clearer."

Speaking to reporters Sunday, Peter Clarke said investigators are also becoming clearer on how the attacks were planned.

Clarke spoke as police in Liverpool searched two houses believed linked to the failed bombings.

Five suspects are in police custody, including three people arrested in northern England late Saturday and early today. Clarke refused to divulge details of the arrests or the nationalities of the suspects.

Police have linked three suspects to two failed car bombings in London Friday. The two remaining suspects are in custody in Glasgow, after crashing their burning vehicle into the main entrance to the Glasgow airport Saturday.

Police in Glasgow carried out a controlled explosion today of a suspicious car parked at the hospital where one of the airport attackers is hospitalized with critical burns. Authorities released no information on the car or its owner.

Prime Minister Gordon Brown told British television today the country faces a "long-term and sustained" threat from extremists linked to or inspired by the Islamic terrorist network al-Qaida.

British authorities raised the country's security alert Saturday to its highest level, critical, after the airport attack and the discovery of the two unexploded car bombs in central London Friday.

Security has been increased for major events, including the Wimbledon tennis championship and a concert today in honor of the late Princess Diana.

Some information for this report was provided by AFP, AP and Reuters.