The letter addressed to her was sent by post and received by the party office "Kamalalayam," on June 6.
It was, however, opened only today.
"A sheet of paper inside the envelope was laced with a type of explosive chemical, usually found in firecrackers and a probe is on," a senior police officer told PTI.
The letter had used abusive language against the BJP leader and a similar letter was received three days ago at her Virugambakkam residence, another officer said.
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Tamilisai said the letter had used "filthy, foul language," which no one could read.
"The letter received at party office appears to be a copy of a letter I got at my residence. It threatens me with dire consequences," she told PTI, adding the letter received at her residence was immediately handed over to Virugambakkam police station.
Recently, police had stepped up security to Tamilisai after she complained of having received a threat call over phone.