In the days before the Manchester Arena attack, Salman Abedi, the man police have identified as the bomber, "made most of the purchases of the core components" of the weapon himself and was largely alone as he moved about the city, British police say.

"Many of his movements and actions have been carried out alone during the four days from him landing in the country and committing this awful attack," Detective Chief Superintendent Russ Jackson of the North West Counter Terrorism Unit said, referring to Adebi's recent return from a trip to Libya.

"With specialist support, we have also had a good understanding of the likely component parts of the bomb and where these came from," Jackson said.

When the bomb detonated at the end of an Ariana Grande concert on May 22, it killed at least 22 people and wounded dozens.

Police are still trying to find the distinctive blue suitcase Abedi was seen carrying as he moved around Manchester's city center and other areas ahead of the attack. The suitcase is "a different item" than what was used to carry out the bombing, Jackson said — but he added that people should be cautious and contact police if they can help track it down.

"We are especially keen to find out why he kept going back to the Wilmslow Road area, and we need to find the blue suitcase which he used during these trips," Jackson said.

Officials have not ruled out Abedi's acting as part of a wider network — a crucial question that arose last week because of the sophisticated and effective nature of the attack and the timing of his visit to Libya.

Last week, a high-ranking Western government official told NPR, "Our assessment is he did not have the skills to build such a bomb, and the conclusion is there is a bomb-maker out there."

Manchester police have made at least 16 arrests in connection to the attack and were still holding 11 men in custody, according to the department's latest updates.

As we have reported, Abedi's father and brother were also detained in Libya last week. Another brother was arrested in England.

The latest police findings are the results of painstaking work, Jackson said, with more than 1,000 officers working on the case and helping to track Abedi's movements through security camera footage, his phone calls and his interactions with people.

Calling the scale of the investigation "enormous," Jackson said there are more than 7,000 entries listed on the counterterrorism unit's lines of inquiry. Investigators are talking to hundreds of witnesses and looking at "almost 300 pieces of digital equipment, including phones," he said.

"There has been huge progress made over the week, and the speed of the inquiry remains the same," Jackson said.

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