The investigators, who are looking into the use of chemical weapons in Syria, did not indicate who had deployed the chemical weapons in a report released Thursday to U.N. Secretary General Ban Ki-moon.
The same team reported in September it found "clear and convincing evidence" sarin gas was used against civilians, including children, in the Damascus area.
The report said the team "collected credible information that corroborates the allegations" chemical weapons were used in Khan Al Asal in March against soldiers and civilians.
"However, the release of chemical weapons at the alleged site could not be independently verified in the absence of primary information on delivery systems and of environmental and biomedical samples collected and analysed under the chain of custody," the report added.
The Syrian government has acknowledged it possesses chemical weapons and has promised to eliminate them. The United Nations and the Organization for the Prohibition of Chemical Weapons is overseeing the destruction of Syria's stockpiles.