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The son of a US federal judge has been shot dead and her husband injured in an attack at their family home in New Jersey.

Investigators said the judge, Esther Salas, was unharmed.

According to reports the attacker was dressed as a FedEx delivery driver and is still at large.

The motive for the attack is unclear. A local mayor and family friend said Judge Salas had received threats in the past.

The FBI says it is investigating and has appealed for information.

Sources quoted by US media said Judge Salas's 20-year-old son Daniel Anderl - a student - opened the door at the family home in North Brunswick at about 17:00 on Sunday and was fatally shot. The judge's husband, Mark Anderl, 63, was then shot several times. He is in a critical but stable condition in hospital, the New Jersey Globe reported.

North Brunswick Mayor Francis Womack told ABC News that "as a judge, she had threats from time to time but everyone is saying that recently there had not been any".

Investigators said prior threats made to the judge were a line of inquiry but authorities were also considering whether the husband's work as a criminal defence lawyer could be a motive.

In a statement, the FBI said it was looking for one suspect and asked anyone who thought they might have relevant information to call them.

"We are working closely with our state and local partners and will provide additional updates when available," it added.

New Jersey Senator Bob Menendez said he hoped that "those responsible for this horrendous act are swiftly apprehended and brought to justice".

"I know Judge Salas and her husband well, and was proud to recommend her to President Obama for nomination to NJ's federal bench," he said. "My prayers are with Judge Salas and her family."

Judge Salas serves on the federal bench in New Jersey, the first Hispanic woman to do so, ABC News reported.