The FBI has confirmed to Fox News on Friday that it has joined the search for Veronica Butler and Jilian Kelley, the two women from Kansas who vanished in Oklahoma as they were traveling to pick up children. 

A spokesperson for FBI Oklahoma City said the agency is "providing additional resources to the investigation, but the Oklahoma State Bureau of Investigation remains the lead agency." 

Butler, 27, and Kelley, 39, were last seen on March 30 heading to pick up children before their car was found abandoned near the Oklahoma-Kansas border, with "foul play" suspected, police say.  

As of Friday, there have been no arrests in the case and no suspects or persons of interest have publicly been identified by law enforcement. 

YOUTUBERS SEARCH FOR MISSING KANSAS WOMEN INSIDE ABANDONED HOME 

Veronica Butler and Jillian Kelley vanish in Oklahoma

Veronica Butler, left, and Jilian Kelley were last seen on March 30 while driving to pick up children, police say.  (Oklahoma Highway Patrol)

"We are still telling everybody in that area that if you know anything or have any additional information to please contact us," Oklahoma State Bureau of Investigation public information manager Hunter McKee told Fox News Digital on Thursday. 

McKee told Fox News Digital on Monday that "it’s becoming more and more challenging" to find the women alive as the days mount with no signs of their whereabouts.  

A missing persons advisory from the Texas County Sheriff’s Office said that Butler has "several tattoos, a Chinese symbol on her left forearm, a sunflower on her left shoulder" and that "Jilian has a butterfly tattoo on her left forearm."      

‘MORE AND MORE CHALLENGING’ TO FIND MISSING KANSAS WOMEN ALIVE, INVESTIGATORS SAY   

Veronica Butler and Jilian Kelley missing

Hugoton Assembly of God Pastor Tim Singer tells Fox News that the Jilian Kelley, left, and Veronica Butler, right, were heading to pick up Butler's children to bring them back to a birthday party in Hugoton, Kansas. (Texas County Sheriff's Office/Oklahoma Highway Patrol/Shutterstock)

The alert describes Butler as being 5 feet, 4 inches tall with red hair and green eyes, last seen wearing a blue short-sleeve shirt and denim shorts.      

Kelley has brown hair and blue eyes and was last seen wearing a long-sleeve shirt, blue jeans and tan or beige shoes, the alert adds.      

Remote road in Oklahoma

The area near the Oklahoma-Kansas border where the vehicle the two women were traveling in was found abandoned on Saturday, according to the Oklahoma State Bureau of Investigation. (Google Maps)

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The vehicle they were traveling in was "found abandoned near Highway 95 and Road L, south of Elkhart, Kansas, in rural Texas County," according to the OSBI.