Metropolitan Police officers have discovered what is "believed to be a CS gas spray" or tear gas spray, a Met Police spokesperson said.
"At this early stage, officers are investigating whether it may have been discarded by a passenger prior to check-in," the spokesperson said.
About 500 people were evacuated from the east London airport after some passengers felt unwell. Two people were taken to hospital and 26 others treated at the spot.
The Met have said they are not treating the case as terrorist-related.
Also Read
A London City spokesperson said, "We apologise to passengers for the inconvenience caused today when an alarm was activated, triggering a full evacuation of the airport terminal".
"Passengers were evacuated safely and we thank them for their patience. Following the evacuation, some individuals reported feeling unwell and were treated at the scene by London ambulance service," the spokesperson said.
Emergency services responded to the evacuation, citing a possible chemical incident, with firefighters and police officers jointly conducting sweeps of the airport building.
"The airport was declared safe and reopened later. Passengers are advised to contact their airline for the latest information regarding their flights," officials said.
The closure of the airport led to travel chaos as all flights were suspended. Several incoming planes from destinations such as Frankfurt, Amsterdam, Belfast City and Paris were diverted to other airports.
Disclaimer: No Business Standard Journalist was involved in creation of this content