Unidentified persons hurled steel bombs at the Communist Party of India (Marxist) (CPI(M) ) committee office in Kozhikode, Kerala, on 8 June's night.
The incident took place at around 12:45 a.m., but there was no report of damage or injury.
A steel or pipe bomb is usually a short section of a steel water pipe containing the explosive mixture and closed at both ends with steel or brass caps. A fuse is inserted into the pipe with a lead running out through a hole in the side or capped end of the pipe. The fuse can be electric, with wires leading to a timer and battery, or can be a common fuse.
All of the components are easily obtainable.
Responding to the attack, the CPI(M) has called for a strike.