Police revealed they had found the 'seat of an explosion' and debris near Wolverhampton Central Mosque.
The mosque had been dramatically evacuated while searches were carried out last night, with investigations running into today.
The two Ukranian men, aged 22 and 25, remained in custody and were assisting police with their inquiries this afternoon following their arrest yesterday in relation to blasts in Tipton and Walsall.
How Wolverhampton Central Mosque drama unfolded
Officers today revealed that the Wolverhampton device – which had been placed on the Five Ways Island near the Waterloo Road mosque – exploded at around 9.15am on June 28, having been put there on that morning or the night previously.
Wolverhampton Central Mosque was evacuated at 8pm last night when police acted on fresh intelligence.
They had earlier swooped on an industrial park in Birmingham to arrest the two men.
West Midlands Police Assistant Chief Constable Marcus Beale confirmed officers had responded to reports of a 'bang' on June 28.
He said today: "There was a call about a loud bang and a puff of smoke around that time and there was a normal response officer sent to the scene.
"Now, as a result of intelligence following the arrest of two men in Birmingham yesterday, we have carried out a detailed forensic search of the Wolverhampton scene.
"We have discovered it was the scene of an explosion that took place on June 28."
He added: "We are now investigating this as part of a series of events."
Live updates from the scene at Wolverhampton Central Mosque
Mr Beale said the 'series of events' included the blasts outside the Walsall mosque on June 21 and the Tipton explosion on July 12. Police said they had no information to suggest there were any further specific threats to the region.
Mosque spokesman Iftikhar Ahmed, 51, said worshippers were concerned at the developments but added: "We will have upwards of 500 people here for prayers today, and that is our act of defiance to whoever perpetrated this. It is business as usual for the mosque."
Yesterday's fresh information sparked a major police operation as officers searched the mosque and surrounding areas.
It reopened this morning after police declared the area safe.
Military bomb disposal and specialist search teams had also descended on the mosque.
In Birmingham, parts of the city had earlier been brought to a standstill while hundreds of workers were evacuated from software engineering company Delcam – where the two suspects were arrested at 2.50pm. Detectives, an Army bomb disposal unit and police dog team carried out investigations late into the night.
The first man was arrested after being spotted by officers working in the local area, and the second was arrested nearby.
It is believed an officer on routine patrol spotted one of the men and initially identified him as a suspect in the Walsall mosque bomb inquiry.
The pair are university engineering students and are on work placement with Delcam.
Clive Martell, chief executive of Delcam, said: "We had two men arrested on our premises.
"They are both Eastern European and they are here as overseas students on work placement." The message from mosque leaders was one of calm as they came to terms with the day of drama.
Police reveal details of the arrests at the scene
Prayers were redirected from the Waterloo Road mosque to nearby Dunstall Hill Community Centre, where senior mosque member Mahmood Khan said relations with police and the community were extremely positive.
The first terror investigation was launched after a handmade device exploded by the Aisha Mosque and Islamic Centre in Rutter Street, Caldmore, Walsall, on June 21. The road was sealed off and around 150 residents evacuated.
On July 12, counter terrorism police, the Army and bomb disposal teams were sent to the Kanz-ul-Iman Muslim Welfare Association Central Jamia Mosque in Tipton after nearby residents heard a loud bang.