Entebbe, Uganda -- Four men allegedly involved in the kidnapping of an American tourist in Uganda were under arrest on Tuesday morning, CBS News has learned. A fifth suspect was still on the run.

Kimberly Sue Endicott and her safari guide were released on Sunday after some ransom money was paid, almost a week after armed gunmen abducted them inside the Queen Elizabeth National Park. Endicott arrived at the U.S. Embassy in Uganda on Monday and was on her way home.

CBS News correspondent Debora Patta reports that the Ugandan safari adventure was always high on Endicott's bucket list, but she never imagined it would turn into such a nightmare.

Sources have told CBS News that Endicott and her guide were forced to walk across the border from Uganda into the Democratic Republic of Congo, where they spent most of their five days in captivity.

The pair travelled long distances on foot most days, and slept out in the open bush at night. They were given a mattress and sheet to sleep on, sources have told CBS News.

Endicott's emotional state was up and down during the ordeal, but was apparently at its lowest when they realized they'd crossed the border into the DRC.

Her guide Jean Paul Mirenge Remezo has remained at the wilderness lodge in Ishasha, where Endicott had been staying, since his rescue, CBS News producer Sarah Carter reports. He did not go to Kampala with Endicott and was being debriefed by law enforcement at the lodge.

Staff at the lodge said Remezo had yet to be reunited with his wife after the ordeal. They said he seemed quiet and it was clear he had a tough time in captivity.

A photo provided by the Wild Frontiers tour company on April 8, 2019, shows American tourist Kimberly Endicott and field guide Jean-Paul Mirenge Remezo a day after they were rescued following a kidnap by unknown gunmen in Uganda's Queen Elizabeth National Park. Wild Frontiers/Handout

The four suspects arrested in connection with the kidnapping were being interrogated by police on Tuesday. Sources have told CBS News they are believed to be illegal fish traders and ivory smugglers. A fifth suspect is still on the run.

Ugandan police tracked them down with the assistance of FBI surveillance equipment. At one point, as many as 19 FBI agents were in Kihihi — the town nearest the site of the abduction — assisting with the investigation, according to local authorities. The FBI has acknowledged to CBS News that it aided in the search, but disputed the extent and the number of agents involved.

CBS News has been told the kidnappers "made almost nothing" from the ransom paid for Endicott's release, despite their demand for half a million dollars.

On Tuesday, the Ugandan police said in a statement that their "intelligence-led" investigation was "progressing unhindered, with raids and extensive searches in Kanungu district, where the suspects were arrested and the neighboring areas."

Editor's note: This story has been updated.