A bomb placed in a brief case exploded at the Delhi High Court this morning killing 11 people and injuring over 50. The explosion took place in the reception area at Gate no 5, where hundreds of people had gathered to collect passes needed to enter the courthouse. A crater nearly four feet deep established how powerful the bomb was; police sources say the explosive shows traces of ammonium nitrate and combusted completely, signs of sophisticated bomb-making.The Delhi Police will soon release two sketches of the suspects based on descriptions provided by eyewitnesses.This is the second explosion in the Delhi High Court in four months. A smaller bomb in a parking lot in May did not cause any injuries. Police sources say that attempt was a dry run by the terrorists behind today's attack. The incident in May had underscored the need for security cameras. They have yet to be installed at the court's nine different gates. ( First pics of the blast Home Minister P Chidambaram acknowledged that "Intelligence agencies constantly share intelligence inputs with Delhi Police. Intelligence pertaining to threats emanating from certain groups was shared with Delhi Police in July, 2011." ( Watch: Not yet possible to identify who's behind blast, says Chidambaram Two hours after the blasts, he told Parliament that it is not possible at this stage to identify who is responsible for today's terror attack. The National Investigating Agency (NIA) will supervise the inquiry with a special 20-member team.Prime Minister Manmohan Singh, currently in Bangladesh on an official visit, described the attack as "a cowardly act of a terrorist nature." He added, "We will deal with it... we will never succumb to the pressure of terrorism... this is a long war." ( Read An email received by different media organizations including NDTV said that the Harkat-ul-Jihadi (HuJI) takes responsibility for today's blast. HuJI is a Pakistan-based terrorist group. "We take the mail very seriously," said NIA Chief SC Sinha. The email states, "We owe the responsibility of today's blasts at High Court Delhi... our demand is that Afzal Guru's death sentence should be repealed immediately else we would target major high courts and the Supreme Court of India."Afzal Guru has been sentenced to death by the Supreme Court for his role in the attack on the Indian Parliament in 2001. His mercy petition that asks for his sentence to be commuted to life is pending with President Pratibha Patil. The Home Ministry has reportedly recommended that it be rejected.Today's explosion took place at 10.15 am - "rush hour" for the courthouse which usually starts hearings at 10.30 am. ( Watch ) On Wednesday, Public Interest Litigations (PIL) are heard in the High Court, drawing larger than usual crowds. In the seconds after the blast, lawyers and litigants were seen running for help, their blood-soaked clothes clinging to them."I heard a loud noise and smoke coming out following by cries of people," said Narendra Kumar Singh, a guard at a construction site outside the court.By 3 pm, the High Court was back to work to signal that it remains strong despite today's attack. But the conversations in and outside court-rooms were about the tragedy.Lawyer Sanjeev Beniwal rushed his client Mehtab Singh to a Delhi hospital after the explosion. Mr Singh did not survive."He (Singh) was fighting a property-related case at the court. He was getting his pass made when the blast occurred. His son Rohit and my associate Sachin Powa have also been injured. We rushed them to hospital and I just want them to be safe as I could not save Singh," said Mr Beniwal, who was also injured.The government will find itself struggling to provide an explanation for why terror continues to stalk cities with relative ease. On July 13, also a Wednesday, three bombs exploded in crowded markets in Mumbai during rush hour. More than 20 people were killed. ( Watch: Delhi High Court blast - Eyewitness account "The fact that another terrorist attack has taken place in our capital - and so close to Parliament - is very worrying," said BJP leader LK Advani. He also said that he's not sure if today's attack is in any way linked to the fact that this week marks the 10th anniversary of 9/11. Congress leader Rahul Gandhi was heckled when he visited Ram Manohar Lohia Hospital where some of those injured were being treated. Mr Gandhi also made a quick stop at the High Court.Here are the emergency helpline numbers: