Terrorists may have used hard-to-detect plastic-based explosive PETN in yesterday's Delhi High Court blast -- the same substance used in the underwear bomb plot on a 2009 Northwest Airlines Flight 253 and by the 'shoe bomber'.

Sharing details of the "preliminary" investigations, Secretary Internal Security U K Bansal said that traces of pentaerythritol trinitrate (PETN) have been recovered from the blast site.

"But this is the result of preliminary investigation. Rigorous investigations are in progress," Bansal said, indicating that the explosive traces found were not conclusive.

PETN is one of the most powerful explosives and is difficult to detect. Because of its plastic nature, the explosive can easily pass metal detectors. Even bomb-sniffing dogs cannot detect it because of its low pressure molecules.

The explosive allows terrorists to use only small quantities with enormous damages. Even 100 grams of PETN is enough to blow away a car.

Bansal said PETN is "an explosive of choice for terrorists" if they can lay their hands on it.

The remnants of the high court bomb were being analysed in a forensic laboratory, he said.

Colourless crystals
>> The colourless crystals of PETN were used by terrorist Richard Reid, commonly known as the 'shoe bomber', on an American Airlines jet to Miami in 2001.

>> Al Qaeda operative Umar Farouk Abdulmutallab also concealed this explosives in his underwear when he unsuccessfully tried to detonate a bomb in Northwest Airlines Flight 253, also known Christian Day bomb plot, in 2009.

>> Same explosive was also found in two cargo planes from Yemen to the US in October 2010.