Police are investigating an email sent to various media outlets reportedly by a banned Islamist group in Bangladesh threatening to topple "towering buildings" and telling the media not to run anything that goes against what the mail describes as jihad.

The letter sent to Bangladeshi media on Monday was signed by the group Ansarullah Bangla Team, which is allegedly linked with several groups that claimed responsibility for killing four atheist bloggers this year and as well as with al-Qaida's branch in South Asia.

The email contains a six-point directive also asking that women to stay at home.

Dhaka police spokesman Muntasirul Islam said Tuesday that authorities were taking the threat seriously and were investigating.