Mumbai police commissioner Rakesh Maria received a letter allegedly penned by terror operatives, threatening to carry out bombing attacks in retaliation for the Israeli operation in the Gaza Strip, The Times of India reported on Sunday.

The letter, sent on July 25, was signed by the terrorist group Indian Mujahideen (IM).

“We will take revenge for Gaza,” the letter said, according to the Hindustan Times.

“Maria you were lucky in 1993 to have escaped the wrath but this time you will not. Now we are going to carry a blast and it is a challenge for you if you can stop, stop it. Mujahideen,” it continued, referring to a series of 13 bombs that exploded in Mumbai, then called Bombay, and that killed over 250 people. Maria was involved in the investigation and subsequent arrest of suspects in the case.

Maria launched an inquiry into the origins of the letter and raised a security alert.

“We are not taking any chance,” he said. “All police stations, crime branch units and ancillary branches have been alerted.”

However, the paper reported that the police receive threats regularly, and believe the letter may be a prank.

Sunday marked the 20th day of the Israel Defense Forces’ Operation Defensive Edge in Gaza, aimed at stemming rocket fire on Israel and at destroying a network of cross-border tunnels used by Hamas to launch terror attacks. Palestinian officials in Hamas-run Gaza say over 1,000 people have been killed in the fighting; Israel says hundreds of Hamas gunmen are among the dead. Two Israeli civilians and 43 IDF soldiers have been killed.

A series of attacks in Mumbai in November 2008 by members of a Pakistan-based terror group left 166 people dead, including six at the Mumbai Chabad House, which was targeted along with luxury hotels, a train station and a popular cafe.