Gagneja, Vice President of the RSS (Punjab), was attacked at Jyoti Chowk area here when he had gone shopping with his wife. A four-member SIT had been formed to probe the attack.
Later, Kumar told reporters that some powers were out to disturb peace of Punjab and destabilise the state.
"They needed to be brought to book at the earliest," the former Himachal Pradesh chief minister said.
Along with Brigadier (retired) Gagneja, recently there had been attacks on Sikh preacher Ranjit Singh Dhadrianwale, an RSS volunteer in Ludhiana and Mata Chand Kaur, who died.
Meanwhile, Dr G S Wander, chief cardiologist and director of Hero DMC Heart Institute, said "Gagneja is stable but still critical". There was no deterioration in his condition during the last five days and his parameters were normal.
Gagneja was put off the ventilator for one hour today during which he breathed satisfactorily, he added.