India's home minister said Friday that investigators trying to track down those responsible for this week's deadly bomb blast at New Delhi's High Court have turned up some “promising” leads, but nothing more.

Palaniappan Chidambaram told reporters investigators are working non-stop but that none of the leads are conclusive. He also said India is working with other countries to track down the possible culprits.

Chidambaram's comments come the same day Indian authorities got a third email related to Wednesday's attack.

The new email warned of another possible attack in the city of Ahmedabad, the capital of Gujarat state. Chidambaram said officials are taking the threat seriously, though he described the email itself as “amateurish.”

The death toll from Wednesday's blast in New Delhi has risen to 13, with another victim succumbing to wounds sustained in the attack. Four other people remain in critical condition.

Meanwhile, police made an arrest Friday, detaining a man who allegedly emailed the first claim of responsibility. That email, traced to an Internet Cafe in Indian-controlled Kashmir, has been attributed to the group Harkat-ul-Jihad al-Islami.

Five other people were also held for questioning.

On Thursday, the domestic group Indian Mujahideen claimed responsibility for the New Delhi courthouse bombing in a separate e-mail sent to media organizations

All the emails have threatened further violence.

Police have released sketches of two men seen at the New Delhi High Court at the time of the blast. India's National Investigative Agency has offered more than $10,000 in reward money for information on those responsible.

The blast was the first major terror attack in India since three near-simultaneous bomb blasts targeted India's financial capital, Mumbai, in July, killing some 20 people. No one has been arrested in that case, but authorities say they have focused their investigation on the Indian Mujahideen group.

Wednesday's attack was the second explosion at the High Court this year. In May, a small bomb exploded outside the same court, but did not cause any casualties.