By Ronni Berke and Carol Costello, CNN
(CNN) – May 4, 1970: a turning point in America's Vietnam legacy.
That day, Elaine Holstein's son, Jeff Miller, was one of four students killed when Ohio National Guardsmen opened fire during an anti-war demonstration at Kent State University. Holstein is still haunted. "The nightmare is 40-years-old," she says.
Tensions were high. President Richard Nixon had just announced the Vietnam War had expanded into Cambodia and every family with a young man like Jeff had to grapple with the possibility he might be drafted into combat.
Her son, who would have turned 60 this month, had called her before going to the demonstration. "He said 'don't worry about it. I might get arrested, but I won't get my head broken.'"
Later, when she heard about the shootings on the radio, she tried calling Jeff at his apartment. Another student answered. "He's dead," he told her.
That moment marked the end of Holstein's innocence, she says.
"Not only was Jeff killed, but I feel like I was one of the wounded. It was a defining moment. I'm certainly not the person that I was two minutes before I was on that phone."
"I didn't realize there was this kind of division in the country. There were all the divisions of people who were against women's rights, who were against gay rights, but there was this great division between parents and children."
Holstein shared her son's opposition to the war and says she was stunned by the reaction people had to his death. She received thousands of letters, some of them vicious hate mail. "Threatening letters, 'people like you brought up your kids this way and they deserved what they got.' There would be feces in the envelope."
"I don't know what I expected them to do, obviously their sympathies were not with me," she adds.
Now she hopes that others can learn from her painful experience. For one thing, she says, freedom of speech should never be taken for granted – don't assume that only in other countries did people have to watch what they say.
"This was taken for granted, this could not happen. It happened."
Holstein is also worried about the tone in political discourse today. It's a country divided, she says, but in a different way.
"One of the things that frightens me is, I listen to the rhetoric which is so ugly and I don't think they understand what can happen as a result of this," Holstein says. "There are consequences, you can't just throw words around and call people names and stir people up and think that nobody is going to get hurt."