New Delhi, Jul 14 (IANS): The ammonium nitrate-based improvised explosive devices (IEDs) used in the Mumbai blasts were "not crude" and indicated that they were prepared with "some level of sophistication", union Home Secretary R.K. Singh said here Thursday.

"Preliminary investigations indicate the use of IEDs with ammonium nitrate-based explosives (in the Mumbai blasts). The IEDs are not crude. They indicate some level of sophistication," Singh told reporters here, a day after triple blasts in Mumbai Wednesday evening killed 17 people and injured 131.

The union home ministry has been updating the media regularly about the latest on the terror attack.

Chief of the premier anti-terror National Investigation Agency (NIA) S.C Sinha was in Mumbai and was overseeing the probe into the blast case, the home secretary said.

The NIA, he said, has three teams led by an inspector general assisting the investigators from the Maharashtra Anti-Terrorist Squad and the Mumbai police.

Besides, four forensic teams have visited the blast sites and are assisting the investigators, Singh said.