The terror attack outside Delhi high court this morning brings back focus on the investigation carried out by Delhi Police in unsolved cases of explosions in the capital in the past six years.

Government has already handed over the case to National Investigation Agency (NIA) within hours of the explosion, which was a repeat of May 25 when a low-intensity blast rocked the high court.

Today's blast is the first case of terror attack in Delhi to be transfered to NIA, which was set up after the 2009 Mumbai terror attack case.

Police is yet to crack the May 25 explosion outside the high court though the special cell had questioned several people. Investigators had recovered some nail-like shrapnel, a battery-like object and ammonium nitrate from the spot, sources said adding nails and bolts were packed and detonator was attached to the timer.

Sources then said this pointed to the possible involvement of Indian Mujahideen as the crude bomb had the signature of the terror outfit's methods.

Hot on the heels of the May 25 blast, a torn polythene bag containing silver colour powder, pencil batteries and wires discovered outside a women's college today triggered a security scare here.

Besides the May 25 explosion, police is yet to crack the 2006 blast in Jama Masjid, the 2008 Mehrauli blast or the last year's terror attack infront of Jama Masjid.

After the September 13, 2008 blasts which killed 26 people and injured around 70, Delhi had witnessed several minor blasts.

Fifteen days after the September 13 blasts, a low intensity explosion killed a girl in Mehrauli.

A series of mysterious blasts near IIT in south Delhi as also the Jama Masjid blast in 2006 and last year along with the Mehrauli case remain unsolved.

Last year on September 19, suspected Indian Mujahideen terrorists opened fire at a Taiwanese media crew infront of Jama Masjid, injuring two of them.

Hours later, a pressure cooker and a crude circuit were found in the car which exploded near Jama Masjid with police initially claiming that it was not a terror attack despite Indian Mujahideen claiming responsibility for it.

Police believe that the terror outfit hired some local criminals to carry out the attack. However, there was no forward movement in the case.