One man's search for his birth parents lead him to the most notorious serial killer of all time.

It's estimated that about six million Americans were adopted. Every day, thousands of adults begin the journey to find their birth parents.

When Matthew Roberts, 41, finally tracked down his biological mother, he learned that his father was none other than Charles Manson, responsible for the murder of nine innocent people in a murderous rampage in 1969. "It was surreal," says Matthew, who bears an uncanny resemblance to the man serving a life sentence in a California prison. "I didn't want to believe it. I was frightened and angry," he told the Sun.

Roberts' mother Terry, who lives in Wisconsin, met Manson two years before the infamous Tate-La Bianca murders.

In his days as a cult leader, Charles Manson fathered many children by several different women. While a few of Manson's kids, now in their 40s, have gone on to live normal lives, others have never reconciled their lineage. Charles Manson, Jr., the child of Manson and his first wife Rosalie, killed himself in 1993.