Indian police are examing evidence to determine who was behind bombings that killed at least 77 people and wounded hundreds Thursday in the northeastern state of Assam.

Police say the size and complexity of the attack may indicate the work of a fairly sophisticated organization. At least 12 bombs exploded within an hour of each other Thursday in four towns.

Local authorities imposed a curfew for a second day Friday after angry crowds clashed with police in the state capital Guwahati. They accused the authorities of lax security.

A local television station says it received a cellphone text message (SMS) from a little-known group claiming responsibility for the attacks.  The station News Live said the group calling itself the "Islamic Security Force - Indian Mujahideen" also warned of future attacks.

Authorities have not verified the authenticity of the message, and say their investigation continues.

The group's name is similar to one that claimed responsibility for three deadly bombings earlier this year in Jaipur, Gujarat and New Delhi.

Assam's main separatist group, the United Liberation Front of Asom (ULFA) denied any involvement in the bombings.

Separatist violence in northeastern India has killed some 20,000 people since 1979.  The region also has been plagued by ethnic conflict and attacks that authorities blame on Muslim extremists based in neighboring Bangladesh.  

Some information for this report was provided by AFP, AP and Reuters.