London police checking if terror suspects have link to ISIS

LONDON -- London police are questioning four terrorism suspects believed to have been plotting an attack inside Britain and checking a possible link to the Islamic State of Iraq and Syria (ISIS).

Officials said Wednesday that a medical student is among the group of four suspects, which includes one man who had traveled to Syria. The men, aged 20 and 21, were arrested Tuesday morning in London. They remain in custody and haven't been charged.

Police commissioner Bernard Hogan-Howe told BBC radio the arrests involve a "quite serious case." He said police are taking a more aggressive stance because of the increased threat linked to the fighting in Iraq and Syria.

Armed police from a specialist unit took part in one of the raids, indicating that the situation was deemed potentially dangerous to officers.

Police said no shots were fired when the search warrant was executed.

The terror threat in the U.K. was recently raised to "severe" because of the increase in the number of Britons traveling to Syria to join up with ISIS, the militant group that has taken over the northern part of Iraq,

The government has said the militant group plans to strike targets inside Britain.

It is estimated that between 1,500 and 2,500 Sunni extremists from Europe have traveled to Syria since January 2012 to join groups like the Nusra Front and ISIS, estimates Seth Jones, a former U.S. counter-terrorism official.

The sharp spike in the number of Western jihadis has triggered real concern among leaders in the U.S. and Europe.