Diphosgene (ClCO2CCl3), or trichloromethyl chloroformate, is a colourless, synthetic substance developed for use in chemical warfare during World War I due to its effectiveness in destroying the filters found in period gas masks. Although both sides used it, it was first deployed by the Germans during May of 1916.
Once deployed, diphosgene quickly breaks down into the gaseous forms of phosgene (CCl2O) and chloroform (CHCl3). Although contact may cause irritation of the skin or eyes, disphosgene is primarily a choking agent, and is considered dangerous when even low quantities are inhaled.
Following unprotected exposure, subjects suffer the effects of phosgene inhalation, namely massive pulmonary edema or asphyxiation, within the next 2 to 24 hours. This is caused by the reaction of the phosgene with moisture in the respiratory tract, which creates carbon dioxide and hydrochloric acid. Chloroform levels are considered too low to be of any serious concern.
While chloroform and phosgene have industrial uses, diphosgene has no such use, and is not a normal byproduct of any process involving either. Therefore, any presence of diphosgene can be assumed to be deliberate.
Sources:
Wikipedia entry on diphosgene - http://en.wikipedia.org/wiki/Trichloromethyl_chloroformate
Federation of American Scientists' Special Weapons Primer: Chemical Warfare Agents - http://www.fas.org/nuke/intro/cw/agent.htm
BOC Gases' MSDS on phosgene - http://www.boc.com/gases/pdf/msds/G067.pdf
eMedicine Journal, CBRNE - Lung-Damaging Agents, Diphosgene - http://author.emedicine.com/EMERG/topic906.htm