The letter was seized during nationwide raids conducted by the Pune police in connection with the case following which these activists were arrested, he said.
The Pune police recently filed its charge sheet against five activists -- Surendra Gadling, Shoma Sen, Mahesh Raut, Rona Wilson and Sudhir Dhavale, who were arrested in June.
The letter in question, dated September 25, 2017, is addressed to Surendra (who police claim is Surendra Gadling) by one comrade Prakash, the official said.
A portion of the letter reads, 'We must intensify nationwide protests using students. State forces will be soft against students which will gradually put the state at a disadvantage while acting against us.
'Congress leaders are very much willing to assist in this process and have also agreed to fund further agitations.... In this regard, you can contact our friend at (the phone number of Digvijaya Singh as per police).'
A senior police official said the number in the letter is also available on the website of the Indian National Congress.
A senior official associated with the probe said the letter had been made part of the chargesheet and is under investigation.
Asked if Singh would be summoned for questioning, Pune Deputy Commissioner of Police (Zone I) Suhas Bavche said several numbers had cropped up in the letters seized during the raids and the ownership of these numbers and their roles in the case are being probed.
"If someone's role is established, whatever is needed to be done as part of the investigation, will be done," Bavche said.
Shivaji Bodakhe, Joint Commissioner of Police (Law and order) informed that, as of now, no notice had been issued to any one.
Digvijaya Singh had denied the allegations when they first surfaced and had dared the government to arrest him.
Talking to reporters in Satna in Madhya Pradesh on September 4, Singh had said, "If I am guilty, I challenge the Centre and the state government to arrest me."
The other activists arrested in the case are Telugu poet Varavar Rao, Arun Ferreira, Vernon Gonsalves, Sudha Bharadwaj. While Gautam Navlakha has been booked, he not taken into police custody as yet.
The prosecution had on Sunday told court that charges related to 'sedition and waging war against the state' had been added against all the accused arrested for Maoist links in connection with the Elgar Parishad.
Government pleader Ujjwala Pawar had told court on Sunday that Sections 124 (A) (sedition), 121 (waging or attempting to wage war) and 121 (A) (conspiracy) of the Indian Penal Code (IPC) had been added to the chargesheet.
The Elgar Parishad was held on December 31, 2017 in Pune to commemorate 200 years of the 1818 Koregaon Bhima battle in which East India Company forces, comprising a large number of Dalit soldiers, defeated the Peshwa army.
Digvijay dares PM, Rajnath, Fadnavis to act against him
Senior Congress leader Digvijay Singh dared Prime Minister Narendra Modi, Union Home Minister Rajnath Singh and Maharashtra Chief Minister Devendra Fadnavis on Monday to take action against him in the Elgar Parishad case.
Talking to reporters, Singh said, “If Digvijay Singh is involved in any anti-national activity, then Modiji, Rajnathji and Fadnavisji are free to take action. Show the courage to take action.”
Speaking about the phone number in the letter, the Congress leader said, “The phone number they are talking about has been in the public domain through the Rajya Sabha portal. Thousands of people know that number. I stopped using it around four years ago.”
Singh claimed that the Bharatiya Janata Party and the Rashtriya Swayamsevak Sangh were afraid of him and therefore, vitiating the atmosphere against him.
“I have neither feared them, nor will I be scared of them in the future,” he said.
“Earlier too, the Madhya Pradesh chief minister (Shivraj Singh Chouhan) had termed me anti-national. I had surrendered before the Madhya Pradesh police, which gave me in writing that it had no proof against me, nor was there any case registered against me,” Singh added.
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