An anti-feminist lawyer suspected of killing the son of a US federal judge before taking his own life is believed to have been involved in the murder of a rival men's rights activist, the FBI said Wednesday.
Roy Den Hollander attacked the New Jersey home of Judge Esther Salas on Sunday, shooting dead her 20-year-old son and wounding her husband -- criminal defense attorney and former prosecutor Mark Anderl.
The body of 72-year-old Hollander, who filed numerous lawsuits targeting women in recent years, was found in New York state on Monday morning. He had apparently shot himself.
The FBI said in a tweet that it had evidence linking Hollander with the murder in California on July 11 of Marc Angelucci, another lawyer who brought cases arguing that men were being discriminated against.
Investigators have not given a motive for the murder but CNN, citing a friend of Angelucci's, reported that Den Hollander held a grudge against him because he had taken on a case that Den Hollander believed was his fight.
The suit challenged a US law that said military conscription was only for men.
The FBI has not provided a motive for Den Hollander's attack on Salas's home but in 2015 she heard a lawsuit that he brought challenging the men-only rule.
According to US media, Den Hollander was not happy with the way Salas had handled the complaint.
Den Hollander dropped the case last year, telling a colleague he had being diagnosed with terminal cancer, the New York Times reported.
Salas was not hurt in Sunday's attack but her husband required surgery, according to local media.
According to CNN, citing law enforcement sources, both shootings were carried out by a gunman dressed as a FedEX driver.
Den Hollander was a self-described anti-feminist lawyer who had sued nightclubs for offering ladies' night discounts and Columbia University for offering courses on women's studies.
On his website, he called on men "to fight for their rights before they no longer have them."
Both he and Angelucci had at some point been members of the National Coalition for Men, which claims to fight discrimination against men.
Judge Salas, 51, was the first Hispanic woman to serve as a federal district judge in New Jersey and was nominated to her current position by President Barack Obama in 2010.