Counter-terrorism investigators are assessing what they believe was a suspicious device found on a London underground train.
Experts decided that a controlled explosion was needed to make the item safe, which was discovered just after 11am on Thursday by train staff at North Greenwich station.
Investigators are now seeking answers to key questions, such as who placed the device on the train, for what reason, and whether or not it was viable.
The United Kingdom is on a heightened state of terrorist alert stoked by fears that an attack by Islamist extremists is highly likely.
But there is also the threat of an attack from dissident Irish republicans, which in May was raised to substantial, meaning a terrorist attack is a strong possibility.
Police said they were keeping an open mind but that “the Met’s counter-terrorism command is leading the investigation because it has the expertise to deal with incidents of this kind”.
Several hours after the discovery of the item it was not being decisively classed as a terrorist incident, nor were investigators sure whether it was linked to an ideological motive.
The alert came after a suspicious item was found on a train traveling eastbound on the Jubilee line at 11am. North Greenwich station was evacuated with people being told to “leave immediately” because of an emergency.
Searches and examinations of the scene were carried out by counter-terrorism investigators as part of the inquiry leading to the closure of the station for several hours.
North Greenwich station serves attractions such as the O2 centre, home of concerts and events, and is south-east of central London.
Police said: “Shortly after 11am on Thursday 20 October, the British Transport Police (BTP) was called to North Greenwich underground station after train staff reported finding a suspicious item on a train travelling eastbound on the Jubilee line.
“Specialist officers from the Met and BTP are in attendance and a controlled detonation has since taken place to ensure the item is safe.”
The O2 arena said a concert scheduled for Thursday evening would go ahead.
The O2 said in a statement: “Following a security incident at North Greenwich earlier today, the station and part of the Jubilee Line remain closed. Following discussions with the police, the O2 remains open and tonight’s Nickelback performance will go ahead as planned.”
Scotland Yard’s counter-terrorism command, known as SO15, undertook the investigation because of the suspicion that the device if triggered would explode or cause a fire. It would also investigate if the motive was believed to be jihadi terrorism, Irish terrorism, or other causes such as animal rights or far-right ideologies.
