On Friday U.S. authorities charged Gregory Dow with sexually abusing young girls at a children's home he ran in Kenya.

The accusations come from a section of federal law that allows the U.S. to have "extraterritorial jurisdiction" over certain sex offenses against minors.

Dow allegedly sexually assaulted four girls between Oct. 14, 2013, and Sept. 13, 2017, at the home in Boito, Kenya.

How did U.S. investigators get involved, and how did they build the case against Dow?

The investigation was prompted by someone who reached out to the Lancaster County district attorney's office in fall 2018, District Attorney Craig Stedman said at the news conference.

The investigation was referred to the FBI and the U.S. Attorney's Office. Assistant U.S. Attorney Timothy M. Stengel has led the investigation and will be prosecuting the case.

FBI investigators trained to interview minor victims of abuse interviewed several children who lived at the Dow Family Children's Home.

Four girls disclosed sexual abuse, according to court documents filed by Stengel. One girl said Dow accused her of having sex with another and told her she had to have sex with him as well, according to the documents.

Witnesses told the FBI they saw young girls being called to Dow's room. They also said there was physical abuse at the orphanage "under the guise of strict discipline," Stengel wrote.

Gregory Dow's wife, Mary Rose Dow, took the four alleged victims and other girls at the orphanage to get birth control devices implanted in their arms without their consent, according to authorities.

"This allowed the defendant to perpetrate these crimes knowing the abuse would not result in pregnancy," Stengel wrote.

Although Gregory Dow fled Kenya, Mary Rose Dow was arrested there on charges of child abuse pertaining to the birth control implants. She spent time in jail and was fined.

In all, the FBI interviewed "dozens" of witness in the U.S. and in Kenya, according to court records.

A grand jury federally indicted Gregory Dow on Thursday. He was arrested without incident in East Hempfield Township early Friday morning, according to U.S. Attorney for the Eastern District of Pennsylvania, William McSwain.

The Dow case was brought as part of Project Safe Childhood, a nationwide initiative to combat the growing epidemic of child sexual exploitation and abuse launched in May 2006 by the Department of Justice, McSwain said.

A recent example of a case based on the same type of offense Gregory Dow is accused of involved an Oklahoma man, Matthew Lane Durham.

Durham was charged with engaging in illicit sexual conduct in foreign places and sentenced in 2016 to 40 years in a U.S. prison. He sexually assaulted three girls and a boy while working at a children's home in Nairobi, Kenya, in 2014.