The new investigation "strongly suggests" that medicines contaminated with the toxins, imported into Gambia, led to the cluster of acute kidney injury among 78 children.
Most were aged under 2, and 66 died between June and September 2022. Four more children have since died, bringing the official toll up to 70.
The report, published in the New England Journal of Medicine on Thursday, reached its conclusions by looking at medical records of patients where available, as well as interviews with their parents and caregivers.
The links between the children's deaths and contaminated medicines first came to light in October, when the World Health Organization sent out an alert saying four cough syrups made by India's Maiden Pharmaceuticals Ltd contained toxic levels of diethylene and ethylene glycol and should be withdrawn.
"This likely poisoning event highlights the potential public health risks posed by the inadequate quality management of pharmaceutical exports," the report said.
Maiden has denied its drugs were at fault for the deaths in Gambia, and the Indian government has said the syrups showed no contamination when it tested them. Production at the factory was stopped in October, but the company is now seeking to restart work.
Since the deaths in Gambia, 201 children have also died in Indonesia, and 19 in Uzbekistan, linked to different manufacturers' contaminated cough syrups.
"This likely poisoning event highlights the potential public health risks posed by the inadequate quality management of pharmaceutical exports," the report said.
There have been a number of poisonings linked to diethylene and ethylene glycol in the past in countries including Haiti and Nigeria, but the report says this is the first known incident when imported medicines were at fault rather than domestically developed drugs.
It also says other evidence, such as the tests of the medicines, the wide geographic spread of the cases, and the fact that the illness did not spread to adults, pointed towards a toxin rather than an infectious agent.
