A day after a powerful bomb blast in the Indian capital, police detained six men for questioning Thursday but appeared to be struggling to identify the group that carried out the attack.

The blast, from a bomb hidden in a briefcase at the entrance to New Delhi’s high court, killed 13 people and injured 74.

As investigators pursued scattered leads based on forensic evidence, witness accounts and e-mails from two groups asserting responsibility, analysts said the attackers could be a loose network of individuals from different cities and not part of a hierarchical, tightly structured organization.

“The terrorists have adopted a new style of operations. They are now adopting a decentralized, unconnected model of individuals or tiny groups, each one being handled directly by a coordinator from outside India. This is why our investigations have been running aground in the past two years,” said Ajay Sahni, director of the New Delhi-based Institute for Conflict Management.

“One person facilitates shelter, another buys a cellphone connection with fraudulent papers, a third one assembles the explosives, and a fourth perhaps plants the bomb,” Sahni said. “None of these people talk to each other.”

Investigators traced one e-mail asserting responsibility for the bombing to an Internet cafe in the disputed Himalayan region of Kashmir. The e-mail said a Pakistan-based group called Harkat-ul-Jihad al-Islami, which also has cells in India and Bangladesh, had planted the bomb. On Thursday, police picked up five men — the owners of the cafe, a manager and two regular patrons — for questioning.

Meanwhile, a TV news station received another e-mail in which an indigenous militant group called the Indian Mujaheddin said it had engineered the attack, choosing Wednesday because it was the busiest weekday in court. The e-mail also warned that the group would strike outside a shopping complex next.

The Indian Mujaheddin was accused of carrying out bombings in several cities in 2007 and 2008.

U.K. Bansal, a senior internal security official, told reporters that the new e-mail “cannot be ignored and will be investigated.” He added, “All the leads are being pursued.” He said anti-terrorism squads and forensic experts from other Indian states are assisting in the probe.

A security official in Kashmir said the first e-mail, which was just two lines long, was sent three hours after the blast by an 18-year-old man from the region. The official said the Harkat-ul-Jihad cell in Kashmir has been weakened in recent years by arrests and surrenders.

On Wednesday, New Delhi police released sketches of two people who they said might have brought the briefcase bomb to the court. Later, police in the northern state of Uttar Pradesh detained a 28-year-old laborer at a bus station based on one of the sketches.

The National Investigation Agency, which is probing the attack, has announced a cash reward of more than $10,000 for any person who offers credible clues about the suspects.

Outside the hospital in New Delhi where many of the blast victims had been taken, there were tearful farewells to the dead. Some distraught relatives complained that the hospitals did not even wrap the corpses in white cloth.

“We had to run from one shop to another to buy plastic bags for body parts and cloth to cover the body with. Is there no dignity even in death?” asked Kulwant Singh, who lost a relative in the bombing.

The Delhi high court resumed work Thursday amid tight security, and a makeshift front desk was set up at a school nearby so litigants could be screened and given security passes to enter.