The probe into the e-mail purportedly sent by banned terror outfit Indian Mujahideen (IM) minutes after the Varanasi blast, appears to be heading nowhere with investigators finding it difficult to establish as to who had sent the mail.

"The culprit had hacked into unsecured WiFi network with a broadband internet connection to send the mail," a crime branch official told PTI.

"Technically, the IP address from where the mail was sent has been traced to one Talreja family in Navi Mumbai. But it is very difficult to know who had sent the mail and using which instrument, due to the technical difficulties. Whether a laptop, a computer or any other instrument was used to hack into Talreja's unsecured WiFi network is also not clear," he said.

The IM is suspected of having hacked into the WiFi network of Akhil Talreja, a resident of Sector 17 in Vashi, and sending the email to media houses, claiming responsibility for the Varanasi blast.

Talreja, a 26-year-old disc jockey, was questioned twice but was later let off by police saying he was innocent.

Sources in Mumbai Crime Branch, which is co-ordinating with the Navi Mumbai Police, said they have sifted through the CCTV footages available with commercial establishments in the Sector 17 of Navi Mumbai to identify if any person was found moving suspiciously in the locality.

"But this has not helped much. The probe has reached a dead end," the official added.

According to police, the logger of the router has been disabled, so there was no trace of who logged and hacked it.

A Gmail ID was used and the mail was written on December 6.

The account has been accessed rarely, police sources said.

Two persons, a two-year-old girl and a woman, were killed and several injured when a low intensity bomb went off at Sheetla Ghat in Varanasi, a place where devotees were to gather for puja on December 7.

An FIR was registered at Vashi Police Station against unidentified persons for hacking into unsecured WiFi network in Navi Mumbai and allegedly concealing the existence of a design to wage war against India.

The offence is punishable with imprisonment of up to 10 years, police said.