"This is very incriminating evidence on which the NIA must act. NIA must take into possession the recorded tapes of the interview of Swami Aseemanand," Singh said.
His remarks came amid a political controversy over a purported "interview" given by Aseemanand to a magazine in which he reportedly claimed that the RSS leadership had sanctioned the "Hindu terror conspiracy" that included the blasts in Samjhauta Express, Mecca Masjid and Ajmer Sharif.
Singh, a known detractor of RSS, at the same time said that he was not saying that all Sangh activists are involved in such acts "but some of the fringe elements are involved".
"Aseemanand has been an RSS activist... The BJP has not contradicted as yet his involvement in the terror act. It is a fact that Sangh activists have been charged in Malegaon, Modasa, Ajmer Sharif and Samjhauta Express blasts.... NIA had chargesheeted such elements.
"It's not for the first time tht Aseemanand has made the statement. Even earlier he had said that all these attacks had happened in the knowledge of the Sangh leadership. But for the first time he has named the RSS chief," the Congress general secretary said.
RSS spokesman Ram Madhav has described as "concocted" Aseemanand's purported interview with the Caravan magazine in which he made the claim that the RSS leadership had sanctioned the "Hindu terror conspiracy" that included the blasts in Samjhauta Express, Mecca Masjid and Ajmer Sharif.
Noting that he had heard the "defence" of the RSS and BJP leaders about how the interview could be done inside a jail, Singh said, "They are questioning the process.... In a number of Supreme Court judgments, judges have held that even evidence collected in an illegal manner cannot be ignored in judicial process."
Singh also attacked Centre's investigating agency NIA for not interrogating Indresh Kumar.
The Congress leader said that he was surprised that despite the fact that the accused, who have been chargesheeted, have "named" Indresh Kumar as a "conspirator", the NIA has not interrogated him.
"I would like to know the reasons from the investigating agency why Kumar was not interrogated even when he was named as conspirator by the accused in all four cases," Singh said.
He also noted that on the contrary the person who was named as "conspirator" in Pune blast was "promptly arrested".
"Why then this kind of discrimination is being shown for Indresh," he asked.
To a question as to who is to be blamed if NIA did not interrogate somebody as it was a Congress-led government at the Centre, Singh said that his party does not interfere in the investigation of such cases as they are for a "fair" probe.
The BJP and its ally Shiv Sena have dismissed the media report about Aseemanand's allegation against RSS as "baseless" and blamed it on Congress' "dirty tricks" department for diverting attention from the real issues before elections.
Dismissing the report, RSS ideologue M G Vaidya said Congress will not benefit by this "false propaganda".
Singh, however, said that investigating agencies and jail authorities of Ambala should "ensure full security and safety to Aseemanand".
"In the interview, Aseemanand has confessed that there were only four people in the meeting in 2005, when the decision to carry out the blasts was taken. Of them, Sunil Joshi has been eliminated. Aseemanand has gone on record. After Joshi's elimination, Aseemanand is the only witness," he reasoned.
RSS ideologue M G Vaidya had on Thursday said since elections are round the corner many such things will crop up.
When asked about the charge, Singh said, "let us not connect the two."
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