British police on Tuesday said they had bailed a man arrested a day earlier at Heathrow airport over the 2010 murder of Pakistani politician Imran Farooq.
A 52-year-old man was arrested on Monday as he arrived in London on a flight from Canada on suspicion of conspiracy to murder Farooq.
He was taken to a west London police station before being released on bail until a date in September, police said.
Counter-terrorism detectives had arrested the man at Heathrow and took him to a police station for questioning.
Farooq, 50, a founding member of the Muttahida Qaumi Movement (MQM), was stabbed and beaten to death in London as he returned home from work in September 2010.
Earlier this month, police searched two residential properties in London as part of the investigation.
Police have offered a reward of £20,000 ($30,700, 23,500 euros) for information that helps lead to a prosecution.
Police said the attack "would have required careful planning and would have required help from other people, some of whom may have provided assistance or information unwittingly".