Police were led to the suspect after capturing the teenage authors of an email sent from an internet cafe in Kashmir, TV station CNN-IBN said, citing sources in the National Investigation Agency, or NIA.
The email claimed responsibility for the blast in the name of militant group Harkat-ul-Jihad Islami, or HuJI.
The Press Trust of India (PTI) news agency also said NIA sources told them a suspect had been detained for questioning. The NIA confirmed last week it was investigating an alleged email from HuJI.
Islamist groups have fought Indian rule of the disputed region of Kashmir for years. Indian officials have not accused Pakistan of involvement in the Delhi attack.
The September 7 bomb at the High Court in New Delhi was hidden in a briefcase packed with nitrate-based explosives. It blew up at a busy entrance where metal detectors were faulty, killing 14 people and wounding dozens.
The government and police suffered a storm of criticism for not preventing the Delhi blast, the latest in a string of attacks. Half a dozen recent attacks remain unresolved including three bombs in Mumbai's gold-buying district in July.
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I don't think Delhi will get justice even this time.