Will County Sheriff Photo

Nearly four years ago, a pretty 20-year-old college student was walking to her suburban Chicago home after a night out with friends only to be killed by a hit-and-run driver.

Since then, the family of Melissa Lech-a striking young woman with a stunning smile-has desperately sought to find out who struck her on a dark road and left her to die. They pooled resources with others in the community to offer a $25,000 reward. Last summer, NASCAR driver Kevin Conway put Melissa's picture on his car to enlist the public's help in finding her killer. All seemed in vain.

Until Sunday.

A stranger rang the doorbell of the home where Melissa's sister, Michelle, lives with her husband. The man, who called himself  Dave,  sat down at the kitchen table and confessed. He told Michelle Lech his conscience had been troubled after seeing "the NASCAR thing." He then described how he struck her sister on that August night in 2008, stopped to check that she was still alive, then got scared and drove away.

"He just came and said, 'I ran over your sister, I ran her over'," the sister told ABC's WLS-TV. According to her,  Dave did not apologize or give his last name. But her husband was able to jot down the man's license plate number. And when he left, Michelle called John Ross, the Joliet, Ill., detective who'd been working the case.

Joliet police used the license plate number to track down the man and arrest him. "He was not surprised," said Ross. "He did not appear to be nervous."

Now, 27-year-old David McCarthy is jailed in lieu of a $1 million bail, charged with causing personal injury and leaving the scene of an accident.

But Michelle Lech is not able to forgive him, saying of the suspect:  "you were just coming for yourself, for your own conscience, and then you left."

In a statement, McCarthy's family said they were "deeply distressed" by the hit-and-run allegations and expressed sympathy for the victim's family.

The NASCAR driver who put Melissa's picture on his car - seen by the alleged suspect - said he is "thrilled" to learn of an arrest in the case. Said Kevin Conway: "I hope this brings an element of closure so they [Melissa's family] can begin the healing process."