British police have released a picture of Manchester bomber Salman Abedi that showed him wheeling a blue suitcase on the day of the suicide attack last week in which 22 people were killed.

British police have asked the public to come forward with information as their investigation continues.

"We continue to track the final movements of Salman Abedi and are particularly interested in his whereabouts between 18 and 22 May 2017," Detective Chief Superintendent Russ Jackson ofthe North West Counter Terrorism Unit said in a statement.

"We believe Abedi was in possession of this case in the days before the attack at Manchester Arena on Monday 22 May. I want to stress that this is a different item than the one he used in the attack," he said.

Meanwhile, British intelligence service MI5 is reviewing its processes following the suicide bomb attack last week, amid questions over how much the authorities knew about the assailant.

MI5 has launched two urgent inquiries into how it missed the danger posed by Abedi, the Guardian newspaper reported.

The BBC said MI5 was alerted at least three times to his "extremist views".

A source told Reuters last week that Abedi was one of "a larger pool of former subjects of interest" whose risk remained subject to review by MI5 and its partners.

A source this morning said that Abedi, a British-born Muslim whose parents were from Libya, was one of 20,000 people who was known to MI5 but not one of the 3,000 people under active investigation.

Home Secretary Amber Rudd declined to comment on exactly what was known about Abedi when asked in several interviews this morning.

But in a highly unusual move for British authorities, she confirmed that MI5 was reviewing its practices.

"I do think it is right that MI5 take a look at their processes to ensure that they work to the best of the possible limits to make sure that we keep people safe," she told BBC television.

MI5, established in 1909 to counter German espionage ahead of World War I, is tasked with protecting British national security.

Most of MI5's work is focused on counter-terrorism and it employs around 4,000 people.

Manchester Attack: The Victims

Earlier, police made a further arrest in connection with the Manchester suicide bombing at a concert by US singer Ariana Grande, the deadliest attack in Britain for 12 years.

A 23-year-old man was held on suspicion of terror offences at an address in Shoreham-by-Sea in the early hours of this morning, according to Greater Manchester Police.

As counter-terror officers made the arrest in the seaside town, searches were also carried 420km away at properties in Manchester and Cheshire.

The operation to dismantle Abedi's network is showing little sign of slowing.

Fourteen men are now being held in custody in connection with the Manchester Arena attack.

The Bank Holiday raids followed a flurry of police activity in Manchester over the weekend, with the arrest of a 25-year-old man in Old Trafford and a 19-year-old man in Gorton in connection with attack.

Police have been working round-the-clock since Abedi killed 22 people, seven of them children, and injured more than 100 in the worst terrorist atrocity since the 7 July bombings in 2005.