A holiday trip to Washington to celebrate more than half a century of marriage has ended in tragedy for a Wisconsin couple – two of three people killed outside the White House by a lightning strike.

The third victim was pronounced dead on Friday evening, and one other is in hospital with life-threatening injuries.

James Mueller, 76, and Donna Mueller, 75, of Janesville, Wisconsin, died of their injuries after the lightning strike on Thursday in Lafayette Park, located directly outside the White House complex, the Metropolitan Police Department said on Friday.

The third victim was a 29-year-old adult male.

A fourth person, a woman, was in critical condition, the police department said. Their identities were not immediately released.

The Muellers were on a trip to Washington DC to celebrate their 56th wedding anniversary, according to their niece, Michelle McNett of Janesville.

“They were a very loving couple,” McNett told the Wisconsin State Journal.

“They were very, very family oriented. I think everyone’s just in shock right now and kind of request privacy.”

The couple had five children, 10 grandchildren and four great-grandchildren, she told the paper.

Officers with the Secret Service and the US Park Police witnessed the lightning strike Thursday night and ran over to render first aid, officials said.

Emergency medical crews were called to the scene just before 7pm and had transported all of the victims to the hospital with “critical, life-threatening injures,” fire department spokesman Vito Maggiolo said.

“We are saddened by the tragic loss of life after the lightning strike in Lafayette Park,” White House press secretary Karine Jean-Pierre said.

“Our hearts are with the families who lost loved ones, and we are praying for those still fighting for their lives.”

-with AAP