The Mumbai, Chennai and Hyderabad airports were put on high alert on Sunday after security agencies received an email saying flights departing from these airports may be simultaneously hijacked, reported The Times of India.

A woman sent an email to the Mumbai police saying she had overheard “six people discussing a plan to simultaneously carry out hijacks from Mumbai, Chennai and Hyderabad airports”, the English daily reported quoting sources. The attack, which was planned for Sunday according to the mail, will have a total of 23 people involved, the report added.

Central Industrial Security Force Director General OP Singh told TOI that security has been heightened at these three airports. Airlines have been asked to thoroughly check passengers’ luggage, reported India Today, adding that gates in Chennai’s domestic and international terminals have been restricted for visitors.