The National Investigation Agency (NIA) has raised to Rs 10 lakh the reward money for anyone providing information on the Delhi high court blast even as the investigators suspect that clone of a SIM card was used to send one of the emails claiming responsibility for the last week's terror strike.

The NIA which is probing the case, has raised the reward from Rs five lakh to Rs 10 lakh to encourage people to share information about the perpetrators of the blast with the investigators, official sources said.

Though the NIA has detained several persons suspecting their role in the blast or sending emails claiming responsibility for it, no one has been arrested formally so far as nothing incriminating has been found against anyone.

Sources said while tracking one of the four emails, the investigators have found that a clone of a mobile phone SIM card with Internet connection was used to send it, which proved to be difficult for the investigators to reach the actual sender of the mail.

"We have detained a person in West Bengal and he is being brought to Delhi for questioning but he may not be the actual sender of the email," a source said, indicating the complexity of the case.

Despite help from the sleuths of the ATS of several neighbouring states and the Delhi Police, the progress in the case is slow so far but investigators believe that some definite progress was expected in the next few days, sources said.