The grandson of prominent anti-gun campaigner Pat Regan has been arrested on suspicion of stabbing her to death.
Mrs Regan, 53, was discovered at the property on Marlborough Grange in the Hyde Park area of Leeds on Sunday.
The mother-of-six started campaigning against gun crime when her son Danny was shot dead in 2002.
The 20-year-old man was being held on suspicion of murder, police said. It is thought he had been arrested earlier in the day over another stabbing.
A 45-year-old railway worker was treated in hospital for a stab wound to his arm after challenging a man for trespassing at Leeds station on Sunday morning, British Transport Police said.
Mrs Regan set up a Leeds branch of Mothers Against Guns after her son Danny, 25, was shot at his home in Haydock, near St Helens, Merseyside in December 2002. His killer has not been found.
She had met government officials to discuss how to tackle the problems of guns and gang-related crime.
Dee Edwards, co-founder of campaign group Mothers Against Murder and Manslaughter, said Mrs Regan would visit schools and give talks about the dangers and consequences of getting involved in crime.
She said: "She made such a big difference to so many people's lives. This is an absolute tragedy for so many people."
Mrs Edwards said she had known Mrs Regan for about six years and described her as the "most incredible woman".
"It's just the most dreadful news for everyone. She's just an amazing woman," she added.
Mrs Regan's friend Sylvester Johnson, 74, said: "She didn't want any young people to get caught up in the trouble that her son got caught up in.
"She was very brave and I don't know of anyone who would have a grudge against her."
In February, Mrs Regan was involved with the launch of a DVD warning about the dangers of gun and knife gang culture.
Speaking at the time, she said: "Things are reaching a critical point. Kids are dying before us, but we shouldn't have to bury our own children."
In the same month, Mrs Regan was at the then Prime Minister Tony Blair's side at a Downing Street summit on tackling problems relating to gang-related crime.
Her campaigning work was also praised by Princess Anne during a visit to the city earlier this year.
Labour minister Hilary Benn, who is the MP for Leeds Central and knew Pat Regan, said he was upset by the news of her death.
"I'm shocked, and greatly saddened to hear of Pat's death," he said.
"She made a huge impression on me and on many people that she met. She was determined to do all that she could so that other mothers didn't go through the experience that she went through when her son, Danny, was killed."
Special Thanks to The BBC