An ongoing investigation conducted by MI5 led to the arrest of four men suspected of conspiring to commit an act of terrorism in the U.K.

Metropolitan officers from counterterrorism held the suspects during raids on their homes based on MI5's substantiated suspicions.

All four men collectively range in age between 18 and 27.Each remains in custody while searches at their homes are still in progress.

A Scotland Yard spokesperson explained that they were held "on suspicion of being concerned in the commission, preparation, or instigation of acts of terrorism.

"These arrests were pre-planned as part of an ongoing investigation by the Met's Counter Terrorism Command and MI5," he continued. "The arrests are linked to activity in the U.K."

Investigators still have not proffered any additional details while the operation continues.The investigation blazes a trail of coordinated sting operations that target suspected ISIS supporters holed up in Britain.

Security in the capital has bolstered in the aftermath of Khalid Masood—an ISIS supporter—killing five people in Westminster with car and knife assaults in March.

Khalid Mohammed Omar Ali, 27, was arrested just last month with knives on his person in Whitehall.Since then, he's been charged for conspiring to commit an act of terrorism in addition to a pair of explosives offenses in Afghanistan back in 2012.

An unrelated case from the same day, however, charged three women with preparation of a terrorist act as well as conspiracy to murder.One of the women was shot in a police raid in Willesden in the process of arresting them.