'My wife left for work at World Trade Center on 9/11 and I never heard from her again'

• Watch the video report by ITV Wales journalist Ian Lang

Charles G. Wolf knew the instant he looked up at the burning North Tower of the World Trade Center on September 11 2001 that his Welsh-born wife Katherine was dead.

The couple lived in Greenwich Village, about a mile from the Twin Towers. Mr Wolf, a private pilot himself, heard the boom of the low flying American Airlines Flight 11 six seconds before it crashed into the building between floors 93 to 99.

Katherine worked on the 97th floor.

Running down to the street in his pyjamas that morning, Mr Wolf looked up at the gaping hole and knew "instantaneously" his wife of 13-years had been killed.

After two decades of grief, Mr Wolf says the 20th anniversary is a "different year" for him.

Following the tragedy, Mr Wolf was at the forefront of the fight for compensation for victim's families and his story is now being told in the new Netflix film Worth, with Emmy winner Stanley Tucci playing Mr Wolf.

He said the film has helped him in his grief for his Swansea-born wife who he "fell in love with in a minute".

Katherine was so near the moment of impact, that Mr Wolf had no body or remains to mourn, something that for many years complicated his grief.

But 20 years on from the attacks, he says his perspective has shifted and he now values the time he had with Katherine over what he has lost.

"My recollection of her is when I kissed her goodbye at 8:06 that morning. She left and I never heard from her again," he tells ITV News.

He continues: "I'm so grateful for having a wonderful relationship with a wonderful person that I was with, rather than being sad about losing her. I became happy about having had her in my life, when it could have never had."

Mr Wolf says he would always be grateful his wife did not suffer.

"She never knew what happened, she never felt the pain, she never felt the fear, she never felt the terror. And I have been thankful for that from the very beginning."

Mr Wolf said while he mourns his wife everyday he "refused to let those terrorists take my life".

"Time heals everything happens. You remember the good things. And then at some point down the line you have that watershed moment that I described earlier, where I am glad I had her in my life," he said.