New Delhi, Sep 10 (IANS): A string of emails claiming responsibility for the Delhi High Court blast has left investigators confused as the terror attack in the heart of the capital remains a mystery, with Home Minister P. Chidambaram Friday saying there were "promising" but "not very conclusive leads" in the probe.

Four emails -- three attributing the attack to the home grown terror group Indian Mujahideen -- have been received since the bombing took place on Wednesday.

Two emails, sent from separate IDs, were received Friday, both warning of another attack - in Gujarat's Ahmedabad city.

The third email warned "that (the attack) will be so cruel that you people will not be able to forget it" for decades.

"If you are willing to know the next attack it is 1,8,5,13,4,1,2,1,4. Till you come to know what it stands for, the next blast will be done."

The email sender identified himself as Ali Saed el-Hoori and the message was sent from the killindia@yahoo.com ID, according to sources. This message was traced to a server located in Moscow.

But the sources said that the email may have been been sent from a proxy server having its address in the Russian capital.

Chidambaram said the message written "amateurishly" was being taken seriously by the investigators.

"The third one appears to be a very amateurish mail," Chidambaram told reporters, elaborating that the numerical was a code for Ahmedabad.

"I am not sure whether it was sent by a serious person. Nevertheless, we are taking it seriously," the home minister said.

In the evening, a fourth message was received by media houses.

The latest email was sent from chotoominani5@gmail.com, the same address from which the second message was received Thursday.

Investigators battling for conclusive leads to crack the third worst terror attack since the November 2008 Mumbai strike arrested a man in Jammu and Kashmir who allegedly sent the first email hours after the deadly explosion.

The first message claimed that the attack was done by the Pakistan-based Harkat-ul-Jehadi Islami (HuJI).

The HuJI email was tracked to an Internet cafe in Jammu and Kashmir's Kishtwar district. It threatened to carry out more such attacks if the death penalty to 2001 parliament attack convict Afzal Guru was not immediately repealed.

Chidambaram said the sender of the HuJI email has been taken into custody for interrogation. "We would like to wait for that report to come."

The Kashmir police identified the suspect as Mohammad Sayeed who has admitted that he had sent the email from Global Internet Cafe in the Kashmir town.

With Chidambaram making it clear that all mails were being treated seriously, the investigators were busy tracking the senders and the places these mails were sent from.

Sleuths led by the National Investigation Agency (NIA) were pursuing scattered leads to crack the case but have not achieved any major breakthrough.

Chidambaram said preliminary findings of the forensic examination indicated presence of nitrate and traces of some other explosives used in the bomb.

"These findings are being reconciled to get a clear picture of the exact nature of the explosive used in the blast," he said, in an indication that the investigators had not conclusively found what explosive was used.

Sources in know of the investigation process told IANS that the forensic reports from four labs have given a different account on the explosives used in the blast.

They have agreed that some nitrate based substance was used for the bomb, but there is no clarity on the exact compound.

The reports, according to sources, have separately suggested the presence of PETN (pentaerythritol tetranitrate), nitrate components and some fuel. But there is no consensus.

Chidambaram admitted that the leads followed so far were "not conclusive". "We found there are promising leads, I can't say they are very conclusive leads. These are being pursued round the clock with help of other agencies, abroad too."

Meanwhile, Delhi Police were preparing a third sketch of a suspect of the bombing and were expected to release it Saturday.

"The sketch will be released tomorrow (Saturday)," said a police officer, adding that the earlier two sketches of the men suspected to have placed the suitcase bomb at the Delhi High Court were based on descriptions of eyewitnesses.

Police, acting on a theory that the attackers might have done a recce inside the court premises, are scanning a few of the close-circuit television (CCTV) cameras within the building. However, they have not been able to find a face to match the sketches.

The reward money for any input on the Delhi blast has also been raised to Rs.10 Lakh from Rs.5 lakh declared earlier.