India is weak in forensic science and has few trained police officers who can handle such data, said the April 2006 cable leaked by WikiLeaks.
The rest of the information is obtained through confessions, mainly by threats or torture, it said.
"Forensics is weak in India - only two DNA labs service the entire country," it pointed out.
"Few police officers outside major cities are trained in safeguarding and exploiting electronic data although this capacity is expanding under indigenous cyber-security training and cooperative training with US government agencies," says the cable posted in The Guardian newspaper.
It says "as a consequence, terrorism investigations and court cases tend to rely upon confessions, many of which are obtained under duress if not beatings, threats, or, in some cases, torture".
These factors, along with a "creaky and corrupt judiciary", contribute to cases lingering in the courts for years.
The leaked embassy cable, however, says that India was a "voracious consumer" of the US State Department's Anti-Terrorism Assistance (ATA) training.
India has "digested 42 courses involving 900 Indian security officials and accepting some USD 10 million in equipment transfers since 1995".
But the "sheer size of India's police, paramilitary, and other security agencies at the state and federal levels guarantees that an abundance of first responders and investigators will be behind the training curve".
New Delhi: Indian security agencies torture prisoners to extract information, partly because forensic science is weak in the country, says a leaked US embassy diplomatic cable.
First Published: Saturday, December 18, 2010, 16:06