A 75-year-old man arrested in connection with a home-made explosive found near a Walsall mosque has been released on police bail.
About 150 people were evacuated from homes around the Aisha Mosque, in Rutter Street, on 22 June.
Residents heard a "loud bang", but police were only called after the device was brought into the mosque.
Officers said a man from Walsall, arrested on Thursday, had been bailed pending further inquiries.
A major investigation is being led by specialist detectives and experts from the West Midlands Counter Terrorism Unit.
A police spokesperson said: "Witnesses say the device exploded outside the perimeter building on the Friday night, but not realising the significance of the "loud bang", they did not report the matter to police.
"It was only when a well-meaning member of the public recovered the debris and brought it inside the mosque the following day were police contacted."
Police would like to hear from anyone who may have seen a white man, of average build, carrying a small bag, walking on his own along Corporation Street from the direction of Wednesbury Road between 22:00-22:30 BST on 21 June.