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Anthony Marshall, son of late philanthropist Brooke Astor, is on trial for allegedly looting her estate.
One of the alternate jurors riding on the train helped catch the suspect.
A court transcript shows Justice A. Kirke Bartley Jr. informed lawyers of the incident during a closed meeting yesterday, according to The New York Times.
The transcript says the jury forewoman, a 44-year-old caterer, suffered bruises and other minor injuries last Thursday.
Bartley said it didn't look like there was any connection between the trial and the attack. The assailant was arrested, but none of the other jurors on the train at the time of the incident have been questioned by the district attorney's office, reports the Times.
The late socialite's 85-year-old son, Anthony Marshall, and a co-defendant deny charges that they took advantage of Astor's mental decline so they could loot her $198 million fortune.