Mumbai: At least three persons were killed and three others injured in a blast in the Rashtriya Chemicals & Fertilisers Ltd factory here on Saturday, officials said. A RCF spokesperson said that the blast at the factory in Chembur area of northeast Mumbai occurred around 4.30 p.m. in boiler No. 2 which had been shut down for inspection and maintenance. The blast killed three contract workers working on the boiler and three others were injured. The injured have been rushed to a local hospital for treatment. The RCF - a public sector 'Mini Ratna' unit - has ordered a probe to ascertain the cause of the explosion. The spokesperson added that two other adjacent boilers have not suffered any damage and all other plants at the Trombay factory are fully safe and operating. Top police, company and rescue officials are at the site to supervise the relief and rescue operations.