Mangok Bol has returned to South Sudan to search for his nieces and nephew who were abducted by militants.
A British government official says both suspects in the brutal killing of a solider were part of previous security services investigations for possible terror links.
The official said he could not provide other details because the suspects may face trial.
Investigations by Britain’s domestic security service, MI5, can include undercover surveillance, phone tapping and communications intercepts.
The British government’s emergency committee has met today after yesterday’s attack by two men near London’s military barracks.
An active duty British soldier was killed. One of the attackers went on video to explain the crime, waiving his bloodied hands with a meat cleaver near the body that lay in the street. Police shot and wounded both suspects.