The Prince of Wales has said he and Catherine are "shocked and saddened" by the death of his former nanny's stepson Edward Pettifer, who was killed in the New Orleans attack.
The 31-year-old was among 14 people killed in a vehicle attack on New Year's Day.
His stepmother Alexandra Pettifer, previously known as Tiggy Legge-Bourke, cared for Prince William and Prince Harry in the 1990s.
Prince William added: "Our thoughts and prayers remain with the Pettifer family and all those innocent people who have been tragically impacted by this horrific attack."
This comes after palace sources said King Charles was "deeply saddened" by the death and has been in touch with Mr Pettifer's family.
In a tribute, Mr Pettifer's family described him as a "wonderful son, brother, grandson, nephew and friend to so many".
The suspect, named as Shamsud-Din Jabbar, is believed to have acted alone in a "premeditated and evil act", the FBI said.
Two improvised explosive devices were also found nearby, according to police.
Some of the injured have been discharged from hospital - but more than a dozen remain, with some being treated in the ICU.
His family said in a statement they were "devastated at the tragic news of Ed's death in New Orleans".
"We will all miss him terribly. Our thoughts are with the other families who have lost their family members due to this terrible attack.
"We request that we can grieve the loss of Ed as a family in private. Thank you."
In a statement, the Foreign Commonwealth and Development Office said it was supporting Mr Pettifer's family.
She became a close companion to the young princes following the separation of Charles and Diana, Princess of Wales.
Ms Pettifer often accompanied the two princes on holiday and, following the death of Diana, she helped them adjust to the loss of their mother.
In 1999, Ms Pettifer - née Legge-Bourke - married security consultant Charles Pettifer at a private ceremony in Wales, which was attended by William and Harry.
