According to a report from the Novo Mesto Police Department, the body was found in the river at the village of Preloka.
A post mortem has been ordered but initial medical examination did not find signs of violence on the body. The investigation continues to establish the drowned person's identity.
The report said that the group apprehended in the Črnomelj municipality this morning were allegedly citizens of Algeria.
Another body was recovered from the border river only a week ago, but a police spokesperson said at the time that the person's identity had yet to be established so it could not be claimed that he or she was a migrant.
The spokesperson also said that Novo Mesto police officers had handled two cases of migrants drowning in the Kolpa earlier this year, on 9 and 10 April, while Croatian counterparts handled another case.
Police data show that the number of illegal border crossings has increased substantially in the first four months this year compared to the same period in 2017; 955 illegal migrants had been apprehended by 22 April, a 246% increase on the same period last year.
