In a statement, California-based North American Punjabi Association (NAPA) alleged that the investigating agency "colluded with the guilty" to let them go scot-free and said the verdict has dashed hopes of people who waited for nearly three decades for justice.
"The special CBI court verdict surprised us as the court acquitted the Congress leader but convicted five other accused -- Mahinder Yadav, Balwan Khokhar, Krishan Khokhar, Captain Bhamal and Girdhari Lal," NAPA Chairman Dalwinder Singh Dhoot said in a statement.
The court let off Kumar in the 29-year-old case in which he was accused of murder and of instigating a riotous mob that killed five Sikhs in Delhi's cantonment area.
Kumar, a former Lok Sabha MP from Outer Delhi who was denied Congress ticket for 2009 elections, still faces trial in another 1984 rioting case.
In a third case, Delhi Police has filed a closure report, saying there was no evidence against Kumar to implicate him.