The last suspect in custody, arrested in Birmingham on suspicion of preparation of terrorist acts, was released today.
"Eleven other people were previously arrested and released with no further action," the Metropolitan Police (Met) said.
Four victims who died as a result of the attack have been formally identified as Kurt Cochran, aged 54, Aysha Frade, aged 44, PC Keith Palmer, aged 48, and Leslie Rhodes, aged 75.
"The inquests into the deaths of the victims of the Westminster terrorist attack opened and adjourned on March 29 under the authority of Senior Coroner for Westminster Fiona Wilcox. A provisional date for the pre-inquest review was set by the coroner as 19 May at the Royal Courts of Justice," the Met statement said.
More From This Section
The inquest into the death of Masood also opened and adjourned on Thursday, under the authority of Wilcox.
A provisional date for the pre-inquest review was also set by the coroner as 19 May, at the Royal Courts of Justice in London.
In total, his attack on Westminster Bridge and the Palace of Westminster, which houses the UK Parliament, lasted 82 seconds.
Masood drove his rented SUV into pedestrians on Westminster Bridge on March 22 before crashing his car into railings and then running into the grounds of Parliament.
Armed with a knife, he stabbed a police constable, before being shot dead by police.
In the attack, five people were killed and 50 were injured. The Islamic State group claimed responsibility for the attack.