A day after the Indo-Pak Home Secretary level talks, where Islamabad wanted to know the progress of the case, the Home Ministry said the National Investigation Agency (NIA) conducted "painstaking" investigation in the 2007 bomb attack case.
The investigators visited a number of states at different locations and also involved experts from the Forensic Science and Railways in their probe.
"A breakthrough in the case was achieved in the month of December 2010 on the arrest of a key conspirator, who confessed not only of his involvement in the criminal conspiracy but also divulged names of co-conspirators, who had caused the blasts. Further investigation is continuing," the Home Ministry said in a statement.
The blasts took place in two coaches of of the Samjhauta Express in the intervening night of February 18 and 19, 2007 near Panipat in Haryana.
Sixty-eight people lost their lives and 12 others injured. Out of 68 who died, 15 bodies could not be identified and out of 53 identified bodies, 43 were of Pakistani nationals. Out of 12 injured persons, 10 were Pakistani nationals.
On Monday and Tuesday, during the Home Secretary level talks, Pakistan wanted to know the progress of the case and the Indian side conveyed that the probe was still on and when it would be completed, the same would be shared with Islamabad.
Right wing activist Swami Aseemanand, who is currently in jail, had in his confessional statement admitted his involvement in the blast and divulged names of co-conspirators who had caused the blasts.
According to the forensic investigation, low intensity explosive material were used in an improvised manner kept in suitcases with incendiary oil kept in pet bottles.
The design of the bombs was such that in addition to the explosion in two compartments which completely got damaged, it also caused huge fire in three other compartments.