UNDATED (USA Today) -- Missouri will become the eighth state Tuesday to enact Kelsey's Law, which requires cellphone carriers to provide law enforcement with a customer's location information in an emergency.

Named for Kansas teenager Kelsey Smith, whose body was found four days after she was abducted on June 2, 2007, the law is intended to ensure local police agencies quickly get what they need to find people in danger.

The law has been gaining ground steadily since the first one took effect in Kansas in 2009. Nebraska, Minnesota and New Hampshire enacted laws in 2010, followed by North Dakota in 2011 and Hawaii and Tennessee earlier this year. A similar bill is in the works in Illinois.

In most cases, the parents of the law's namesake have been heavily involved in promoting passage. Since their daughter was abducted, sexually assaulted and murdered in 2007, Greg and Missey Smith have toured the country telling the story of their agonizing wait while neighbors and friends searched for the 18-year-old.

"They are on a mission," said Jeanie Lauer, a Republican Missouri state representative who sponsored the law in her state. The Smiths gave moving testimony to motivate legislatures to approve the measure, Lauer said. "It is touching to hear them relive it," she said.

Although federal law allows cellphone companies to provide location information to law enforcement in certain circumstances, Kelsey's Law seeks to mandate it.

Catherine Crump, staff attorney at the American Civil Liberties Union in New York, agrees that the information should be made available in emergencies. She compared the practice to an officer who hears a woman screaming inside a home. The officer is allowed to kick the door in to help her.

"If you think there is a true emergency, you can go in," Crump said.

Another provision in the law protects cellphone providers from lawsuits, cutting down on potentially lengthy liability discussions among a company's legal team - as was the case in Kelsey Smith's abduction, said Missey Smith.

"It was 2 o'clock in the morning on a Saturday when they asked for the information," she said. "On Wednesday, we found her." Missey Smith said some estimated it took less than 45 minutes to find Kelsey's body after her cellphone location was finally handed over.

Saralyn Hayes, who oversees emergency communications in some Kansas and Missouri counties, says authorities have been able to act faster in situations in which seconds matter since the law was passed in Kansas.

One young woman was dragged by her hair into a wooded area by an ex-boyfriend, Hayes said. "By the time they located her, it was a pretty scary situation." Without Kelsey's Law, Hayes believes, officers might have been too late.

"No other family should have to wait four days to find out where their loved ones are when the technology is available," Missey Smith said.

Contributing: Rollins also reports for the Springfield (Mo.) News-Leader