Police working on a major counter terrorism operation in Birmingham, the second largest city in Britain, arrested four people on Tuesday, increasing the number of arrests in their anti-terror probe to 12. A brief statement released by West Midlands Police said the men - three aged 19 and one aged 24 - were detained by officers from the West Midlands Counter Terrorism Unit at their home addresses early Tuesday morning. All four are being held on suspicion of fundraising in Britain for terrorist purposes and of traveling to Pakistan to undertake terrorist training, the statement added. The police stressed that the latest arrests were pre-planned and not made in response to any immediate threat to public safety. Detectives have an initial 48 hours to question the suspects before charging, releasing or applying for a warrant of further detention. Eight people arrested in September have already been charged in connection with this ongoing investigation, known as Operation Pitsford. Seven are currently on remand after appearing in court. At the same time, the terrorism threat level in England, Wales and Scotland remains unchanged at \"substantial,\" meaning that a terrorist attack is a \"strong possibility.\"