The boyfriend is suspected of setting the 33-year-old on fire.
The police in Kenyan Trans Nozia, located near the border with Uganda, report that the boyfriend allegedly bought gasoline, poured it over Cheptegei, and set it on fire after a fight on Sunday.
Kenyan media have previously reported that one of the athlete's daughters witnessed what happened.
He kicked me when I tried to help my mom, she says.
I cried for help, a neighbor came and tried to put out the flames with water, but it didn't work.
The boyfriend is also being treated in hospital for severe burns.
Cheptegei finished 44th in the marathon in Paris with a time of 2.32.14.
