According to the Vapi GIDC police, both the units, which have sought secrecy, have received the letters from an unknown source that threatens to ruin them. While Aarti Industries is a chemical manufacturing unit, producing Benzene-based down stream and derivative, the other two are non-chemical units, the police said.

Sub Inspector and Investigating Officer M A Masa said, "In the last week of April, Aarti Industries had lodged a complain following a threat letter. Interestingly, though the letter was sent through post and had a stamp on it, it did not carry the city code. So, it is getting difficult for us to trace the location."

On the latest letters, Masa said the investigation is going on, adding that both the units have refused to disclose their names to avoid inviting any more risk. He said the earlier letter warns of "laying some explosive in the unit."

According to the police, a spark in a chemical unit could lead to a major trouble, as Benzene is known for substituting chemical reactions. Aarti Industries had recently done a mock drill to check its preparedness for unforeseen circumstances, said Masa.