Lankesh was gunned down by unidentified assailants in front of her residence here on September 5.
The 21-member SIT released the sketches, prepared on the basis of information collected from witnesses, and the CCTV footage more than five weeks after she was killed.
"There are only two suspects, but we have released three sketches, based on the accounts of two witnesses...We have released two sketches of one of the suspects because they were drawn by two professional painters,"SIT chief B K Singh told reporters here.
"We have also ruled out any professional rivalry behind Lankesh's killing. In other words, no journalist was involved in it. But, all other angles (including Naxal and personal) are being probed," he added.
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Singh said the SIT was probing the case with "a free mind".
He added that the suspects, aged between 25 and 35 years, had stayed in Bengaluru for a minimum of seven days before committing the crime and carried out a reconnaissance of the journalist's house.
Replying to a query, the SIT chief said the investigators had collected certain clues from the video footage obtained from the house of Lankesh.
"In fact, we began our investigation by collecting evidence from her house on the day of the crime," he added.
Singh said the SIT could not say with a "100-per cent surety" whether the pattern behind the killings of rationalists Narendra Dabholkar and Govind Pansare in Maharashtra, progressive Kannada writer M M Kalburgi in Dharwad, Karnataka and Lankesh was similar.
The SIT chief also refuted reports that they had sought assistance from Scotland Yard in solving the case.
"We will definitely seek assistance from international agencies if we require. In this case, we have not sought any such help," he said.
The footage released by the SIT shows a man riding a motorcycle without a registration number, apparently to conceal his identity.
He added that the sketches were released so that the suspects could be traced and requested the people to share information with the SIT, if they had any.
The officer said members of the public could share information regarding the suspects with the SIT on mobile number 9480800202, WhatsApp numbers 9480800304, 9480801701 or by sending e-mails to 'sit.glankesh@ksp.gov.in'.
Singh added that if the information provided by anyone led to the arrest of the accused, the identity of the informant would not be revealed and he would be "suitably rewarded" as had already been declared by the Karnataka government.
The killing of the journalist, known to be an anti- establishment voice with strident anti-right wing views, had led to national outrage and protests over alleged "rising intolerance" in the country and attempts to muzzle dissent, with the criticism targeted at the central government and right-wing groups.
The Karnataka government has announced a Rs-10 lakh reward for anyone who would provide clues on the killing of Lankesh.
As a political slugfest broke out over the killing of the journalist, her family had made a plea against giving a political colour to it.