Harkat-ul-Mujahadeen is one of the oldest of several insurgency groups that operate inside Indian-administered Kashmir.
The Harkat-ul Mujahadeen, blamed for the Indian Airlines hijacking, has now become one of the most prominent of the militant groups that operate in Kashmir.
The Kashmiri militant group has denied that it is responsible for the hijacking, but has said that it was responsible for raiding a key Indian security installation in Srinagar, the capital of Indian-administered Kashmir.
Harkat-ul-Mujahadeen is one of the oldest of several insurgency groups that operate inside Indian-administered Kashmir.
According to the Indian authorities, the group was formerly known as Harkat-ul-Ansar, but changed its name two years ago after being named as a terrorist organisation by the United States.
Indian security forces say that the group is based inside Pakistan, which helps to arm and train it.
Kidnap
India say that it was Harkat-ul-Ansar - acting under the name of Al-Farhan - who were responsible in 1995 for the kidnapping of six western trekkers in Kashmir.
One of them was beheaded, one escaped and four are missing, presumed dead.
Senior Indian sources say that the tactic of Harkat-ul-Mujahadeen is not to admit responsibility for many of its actions, especially those that are likely to be unpopular in the west.
That is why they admitted carrying out Friday's raid on a key Indian security installation in Kashmir, yet on Monday denied it was responsible for the hijacking of the Indian Airlines plane.
Harkat-ul-Mujahadeen have also denied that the cleric who the hijackers want India to release, Masoud Azhar, is a member of their organisation.
Strictly speaking that could be true, because Mr Masoud is considered to be a preacher rather than an active militant.
But Indian sources in Srinagar say that he nevertheless has close ideological links to the group and is considered to be one of its senior spiritual leaders.