One of two teenagers feared to have crossed into Syria to join ISIS is believed to be a relative of Hammaad Munshi (pictured), who became Britain's youngest convicted terrorist when he was found guilty of terror offences at the age of 18 in 2008 . A relative of Britain's youngest convicted terrorist is believed to be among two  teenagers who have fled into Syria to join Isis. The pair from Dewsbury, West Yorkshire, have not been in contact with their families for several days and cannot be reached on their mobile phones. One of the boys is believed to be a relation of Hammaad Munshi, who was just 15 when he joined a cell of Islamic fanatics targeting the Royal Family. The grandson of a British sharia judge, Munshi was recruited by a leading 'cyber terrorist' who radicalised impressionable Muslims and encouraged them to attend military camps in Pakistan. He was arrested on his way home from a chemistry exam in 2006 and found with two bags of ball-bearings - key material for a suicide vest. He was locked up for two years under the Terrorism Act in 2008. The Times reported that the two 17-year-old boys told relatives that they were going on a school trip, but instead used the Easter holidays as a 'window of opportunity' to flee Britain. They are thought to have boarded a Thomas Cook flight from Manchester Airport to Dalaman in Turkey on March 31, said West Yorkshire Police. Both families have reported the boys missing to police although British authorities now fear that the pair have managed to cross into Syria from Turkey. A family friend said they were in a state 'of total disbelief' and they were 'broken and inconsolable'. The south Dewsbury area has been linked to a number of extremist plots and was home to the ring-leader of the London bombings on July 7, 2005, Mohammad Sidique Khan. According to The Times, a member of one of the families said: 'We don't want to talk.' Assistant Chief Constable Mark Milsom, from West Yorkshire Police said: 'We are extremely concerned for the safety of these two boys and would urge anyone with information to come forward and speak to us. 'Syria is an extremely dangerous place and the public will be aware of the dangers these boys may face. 'The choice of returning home from Syria is often taken away from those that come under the control of Islamic State, leaving their families in the UK devastated and with very few options to secure their safe return.' The force said it has been supporting the missing teenagers’ families and carrying out extensive inquiries alongside the North East Counter Terrorism Unit. The disappearance of the two teenagers follows the arrest of nine Britons by Turkish authorities attempting to sneak into Syria across the Turkish border. Waheed Ahmed, 21, from Rochdale was arrested with his aunt, cousins and four children while attempting to get into Syria. Scroll down for video . 'Seeking jihad': Two teenage boys from Dewsbury, West Yorkshire, are believed to have fled to Syria to join Isis militants, seen here taking control of the Yarmouk refugee camp in Damascus . After the arrest, anti-terror police raided the home of his father, Shakil Ahmed, a respected Labour councillor for the town. Munshi's family comes from Dewsbury in West Yorkshire, where his grandfather Yakub Munshi ran the town's sharia court. The schoolboy, who was studying for his GCSEs at Westborough High School in Kirklees, West Yorkshire, was traced through his online conversations with 'cyber terrorist' Aabid Hussain Khan. In one MSN chat, they argued how best to sneak a metal sword through airport security. Munshi had painstakingly scoured the internet for details on how to make napalm and grenades. He claimed he had only done the research out of curiosity but handwritten notes hidden under his bed revealed his burning desire to travel abroad to become a martyr. The boys are from the same town as London 7/7 suicide bomber Mohammed Siddique Khan (pictured) Munshi also had a library of videos, documents and audio clips showing acts of jihad, mujaheddin fighters and Al Qaeda preachings. He was arrested as he walked home from school after an exam. Khan, who acted as an administrator on an extremist website called Activion Publications, downloaded information on the Queen, the Duke of Edinburgh, Prince Charles and Camilla, Prince Edward, the Countess of Wessex, The Duke of York and the Princess Royal. Munshi was convicted of making a record of information likely to be useful in terrorism. Khan was found guilty of possessing articles for a purpose connected with terrorism but cleared of a similar charge. A third man, Sultan Muhammad from Manningham, West Yorkshire - Khan's cousin - was convicted of three similar charges and one of making a record of information likely to be useful in terrorism.
Two 17-year-olds have not been in contact with families for several days . 'Told relatives they were going on a school trip during Easter holidays' One is 'relative of Hammaad Munshi, who joined Islamic cell aged just 15'