Five persons were killed and 13 others critically injured today in a blast at a chemical factory in Khanna town of Ludhiana district. PTI quoting Police sources reports, two persons were killed on the spot while three succumbed to their injuries later taking the toll to five. The 13 persons who received serious burn injuries were admitted in PGI, Chandigarh and Government Medical College and Hospital Chandigarh. Ludhiana Range DIG M F Farooqi who visited the factory to take stock of the situation said ,Police is investigating the case and stern action will be taken if anyone is found guilty of negligence. Around 50 persons were working in the factory when the blast took place.As per initial reports, the blast took place due to explosion of a boiler in the unit.