Two more people arrested as part of the investigation into the Westminster attack have been released by police.
A 35-year-old man was released without charge and a 32-year-old woman was bailed pending further enquiries. Both had been arrested in Manchester.
The development leaves two people in police custody over the attack - a 58-year-old man and a 27-year-old man, both of whom were arrested on Thursday.
Six others were released earlier and a 39-year-old woman remains on police bail.
- Video report by ITV News Correspondent Paul Brand
The carriage gates at Parliament are usually left open for MPs and their staff to get into work, but Wednesday's attack showed it as a weakness in security.
The Palace of Westminster is surrounded by iron fences and concrete barriers, with airport screening for visitors, but it took just one man running in through the carriage gates to kill a police officer.
The gates were not closed until minutes after the attack - new footage has emerged showing what looks like a courier on a motorbike entering the gates while police were dealing with the attacker.
A committee of MPs had warned last month that the metal barriers were not enough to keep them safe.
The barriers were due to be replaced anyway, but now there will be a full review of security at the palace.
Six people arrested in connection with the investigation into Wednesday's terror attack in Westminster have been released "with no further police action", Scotland Yard said.
A total of 11 people have been arrested since the inquiry was launched but only four remain in custody - two men aged 27 and 58 who were arrested in Birmingham on Thursday and a 32-year-old woman and a 35-year-old man who were arrested in Manchester on Friday.
All are being held on suspicion of preparation of terrorist acts.
The six released without facing further action are two women aged 21 and 26 and four men aged 23, 26, 27 and 28 who were all arrested at addresses in Birmingham.
A 39-year-old woman arrested in east London has been released on bail until late March.