NEW DELHI (AP) — Police arrested two more suspects Sunday but have not identified any other groups involved in coordinated bomb attacks that killed at least 81 people in India’s troubled northeast, officials said.

The arrests bring the number of people detained to five, including two who were taken into custody Saturday because their vehicles — a car and a motorcycle — were believed to have been used in the explosions that rocked Assam state on Thursday. Police gave no details about Sunday’s arrests.

"We are on the track. We are pursuing several leads and hope to crack the case soon as to which groups were behind these attacks," said Bhaskar Jyoti Mahanta, a state inspector-general of police, said by phone from the state capital, Gauhati.

Police also arrested a man whose cell phone was allegedly used by a little-known group, the Islamic Security Force (Indian Mujahadeen), to send a text message to a television station claiming responsibility for the attacks. The message also thanked the group’s partners and warned of future attacks.

Police have said the state’s largest separatist group, the United Liberation Front of Assam, may have been involved in the attacks and is the main target of the investigation, but a ULFA spokesman denied the group had any role.

The ULFA wants independence for India’s northeast, an isolated region wedged between Bangladesh, Bhutan, China and Myanmar with only a thin corridor connecting it to the rest of India. Its residents are ethnically closer to Burma and China than India.

The name of the group that sent the text message echoes that of the Indian Mujahideen, a group unknown until May when it said it was behind bombings in the western city of Jaipur that killed 61 people. It also claimed responsibility for blasts in the western state of Gujarat in July that killed at least 45, and blasts in New Delhi in September that killed 21.

More than 300 people remain hospitalized three days after last week’s blasts and four more succumbed to their injuries, bringing the death toll to 81, a police officer said on condition of anonymity because he was not authorized to talk to reporters.