Police investigating the suspected chemical incident which led to the evacuation of London City Airport have discovered what is "believed to be a CS gas spray", a spokesman said.
The find came after police and firefighters scoured the airport following the alert, which saw dozens of passengers treated for breathing difficulties.
The airport spokesman said it was unclear what had caused the chemical incident, but officers were "investigating whether it was the result of an accidental discharge of the spray".
The Metropolitan Police said it was investigating whether the CS gas had been "discarded by a passenger prior to check-in".
Two casualties were taken to hospital and 25 were treated at the scene, London Ambulance Service (LAS) said.
A spokesman for London Fire Brigade (LFB) said it was called to the airport at 4.11pm to "reports of a chemical incident".
A "full evacuation of the airport terminal" saw around 500 members of the public and airport staff forced to leave.
The airport, which reopened after being declared safe at around 7pm, was said to have closed after an alarm was activated.
Passengers described the situation as "quite scary", with airport staff jumping over check-in desks to escape.
An LFB spokesman said "two complete sweeps of the airport building" were carried out jointly by firefighters and police officers wearing protective equipment.
He added: "No elevated readings were found and the building was ventilated, searched and declared safe."
Medics trained to treat people in hazardous situations also attended the scene.
LAS assistant director of operations Paul Gibson said: "All patients were treated for minor breathing difficulties and two were taken to hospital."
David Morris, who was one of the passengers caught up in the incident, said he was at the check-in desk for his BA flight to Edinburgh when people started coughing.
"We were queuing up and we were just about to check our bags in, and I was talking and started to cough to the point I was not able to keep talking," the 28-year-old told the Press Association.
"It was getting quite bad and we saw other people starting to cough at the same time. The people behind the desk were coughing the most and quite aggressively.
"Within two minutes, they shouted for everyone to get out."
Mr Morris said BA staff behind the check-in desk jumped over to escape, calling the situation "quite scary".
He said that whatever was causing people to cough did not smell or have any colour to it.
Once outside, he said those who had been coughing were sectioned off and checked over by paramedics.
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.
A London City spokesman 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.
"Emergency services responded to the evacuation, citing a possible chemical incident, with firefighters and police officers jointly conducting sweeps of the airport building.
"The search of the airport led to the discovery of what is believed to be a CS gas spray.
"Whilst the cause of the incident has yet to be confirmed, officers are investigating whether it was the result of an accidental discharge of the spray.
"The airport was declared safe and reopened at approximately 1900. Passengers are advised to contact their airline for the latest information regarding their flights."