Ethan Knox was a former employee of Lord Technologies.
Terrorizing National City
On the behalf of his former boss, Knox planted a bomb on an old lab owned by Lord and did several other terrorist attacks on National City, in return for a lifelong doctor for his daughter. His two last attacks were that of National City Airport and Lord's new train. He planted the bomb at the airport and later went to the train, with an explosive jacket.
He then explained to Supergirl that he was doing all the attacks for his daughter's life, and also said that his identity wasn't supposed to be revealed. He then said that she wouldn't save him, but would save the lives of the rest of the people on the train. She then separated Knox's wagon from the rest of the train. Knox's explosive jacket then blew up, killing him.