India\'s top criminal investigation agency on Tuesday opened a probe against five coal companies after raiding premises across the country over the alleged mis-allocation of lucrative mining rights. Central Bureau of Investigation (CBI) spokeswoman Dharini Mishra said that 30 premises had been visited as detectives examined whether coal companies were guilty of cheating in a scandal that has rocked Prime Minister Manmohan Singh. \"We have registered an FIR (First Information Report) after conducting raids in 10 cities,\" Mishra told AFP, adding that coal company offices in New Delhi, Mumbai and Kolkata had been targeted. An FIR is a written report detailing an alleged crime, triggering an investigation. Mishra declined to name the companies involved. The CBI will probe whether some of the firms were set up only to get the coal blocks allocated by the government, and then sold them on at massive profits. An official auditor\'s report last month estimated that since 2004 private operators who won coal blocks without competition may have enjoyed \"financial gains to the tune of 1.86 trillion rupees ($33.4 billion)\". Singh\'s coalition government, led by the left-leaning Congress party, has been beset by a string of corruption cases since re-election in 2009 and the latest allegations of mismanagement have led to renewed pressure on him. The Hindu nationalist Bharatiya Janata Party (BJP) and other opposition parties have forced parliament to be adjourned daily over the issue and have demanded the prime minister\'s resignation. Singh, who along with being premier was also in charge of the coal ministry when the mining rights were allocated, has strongly rejected charges over the coal scandal, saying the auditor\'s findings were not supported by facts.