French sports daily L'Equipe said Monday that the World Anti-Doping Agency-accredited laboratory in Paris found a banned diuretic in her urine sample taken July 6.
That day, Selsouli timed 3 minutes 56.15 seconds at the Paris Diamond League meeting and became a medal favourite for the Olympic event.
Athletics' governing body, the IAAF, said Monday that it "cannot confirm anything at this time" about the doping allegation.
Selsouli finished serving a two-year ban last August for using EPO before the 2009 world championships.
The 28-year-old runner could be banned for life if a second doping offence is proven.
Selsouli's sample contained traces of furosemide, L'Equipe reported. The substance is included on the WADA prohibited list, and can be used as a masking agent to disguise the presence of other performance-enhancing drugs.
The IAAF declined to confirm if Selsouli has been provisionally suspended and ruled out of the London Games.
However, British distance runner Paula Radcliffe posted a message on Twitter suggesting she expected Selsouli to miss the Olympics.
"Good riddance, no surprise," wrote Radcliffe, the women's marathon world record holder.
Selsouli returned from her first doping ban to win silver in the 1,500 at the indoor worlds in March at Istanbul.
Her run in Paris this month sliced more than four seconds off her previous best time.