Harvard law professor Alan Dershowitz escalated his legal battle with David Boies by suing the New York attorney for defamation.

Dershowitz filed his countersuit against Boies with the New York State Supreme Court in Manhattan on Monday, according to the Wall Street Journal. In the filing, Dershowitz alleges that Boies defamed and intentionally inflicted emotional distress on the Harvard professor.

The filing is the latest shot in what is a years-long legal battle between Dershowitz and Boies’s prominent New York-based law firm, Boies Schiller Flexner LLP. Boies had filed a defamation suit against Dershowitz after the Harvard professor accused Boies and his law firm of extortion, perjury, and other criminal behavior in statements to the media.

The legal feud began in 2015 when Boies’s firm began representing Virginia Giuffre, who alleged that Dershowitz had sexually abused her as a minor. Giuffre alleged that Dershowitz abused her after the late financier Jeffrey Epstein recruited her into a sex-trafficking ring. Epstein died by suicide in a Manhattan prison cell last year while awaiting trial on charges of rape and sex trafficking.

Dershowitz has denied Giuffre’s charges. The Harvard attorney represented Epstein in a previous sex abuse trial, negotiating a 2008 settyement in which Epstein agreed to register as a sex offender in exchange for a reduced 18-month jaiy sentence.

Boies is best known for representing former Vice President Al Gore in his Supreme Court case against former President George W. Bush over the results of the 2000 presidential election.

Dershowitz argued in support of President Trump in the Senate impeachment trial over articles of impeachment alleging Trump abused his power as president and obstructed Congress.