(AP) — In 2018, the FDA began investigating whether the increasing popularity of grain-free dog foods had led to a sudden rise in a potentially fatal heart disease in dogs.

Four years later, the FDA has reached no conclusion, but the publicity surrounding the issue has shrunk the once-promising market for grain-free dog foods.

A six-month investigation has found that a tangled web of industry funding and interests influenced the origin, data collection, and course of the FDA study.

Makers of grain-free dog foods and their suppliers, in turn, have exerted pressure on the FDA to protect their market share.

100Reporters conducted the investigation that indicates, according to its website, that veterinarians who “prompted the FDA to consider diet have financial and other ties to the leading sellers of grain-exclusive foods.”

It also claims some veterinarians were told to submit cases that implicated grain-free food for FDA study.

Click here for the entire 100Reporters investigation.