"It was a devastating attack on the mosque as people gathered for prayer", said Al Jazeera's Ahmed Idris, reporting from Nigeria's capital, Abuja.
He said this is the deadliest attack on civilians by the militant group since the start of the year, "It is deplorable that the attack was launched at a time when civilians were congregating for dawn prayers".
The police spokesman said the young man detonated his explosives Tuesday while mingling among the worshippers. Given the seriousness of the injuries, the death toll was expected to rise, he said.
There was no immediate claim of responsibility, but armed group Boko Haram carries out regular attacks in the region.
"This is obviously the work of Boko Haram".
Yan St-Pierre, a counter-terrorism specialist at the Modern Security Consulting Group in Berlin, said the bombing fitted a pattern of previous attacks.
Adamawa, the state where Mubi is located, is no stranger to Boko Haram's violent attentions, according to NPR's Ofeibea Quist-Arcton.More news: More than 70 Connecticut Foster Kids Adopted by New Families
This could also explain two videos put out on social media by Boko Haram in the last week, purporting to show its fighters in combat and dead Nigerian soldiers, he added.
"They are feeling the heat".
But it has been peaceful since the military and the civilian militia ousted them from the town, which is a commercial hub and home to the Adamawa State University.
Boko Haram held territory in Adamawa state in 2014.
He said, "Today's attack appears to be yet another example of Boko Haram's continued unlawful disregard for human life".
Boko Haram has been blamed for more than 20,000 deaths during its eight-year-old insurgency.