A former Manchester United steward has been jailed for a minimum of 24 years for the Islamic State-inspired murder of a “gentle, well-respected” former imam in Rochdale.

Mohammed Syeedy, 21, was found guilty of helping to kill Jalal Uddin because the 71-year-old practised a form of Islamic spiritual healing considered to be “black magic” by some extremists.

On hearing the jury’s verdict, Syeedy put his head in his hands and shook it in disbelief. In the public gallery, his family gasped and sobbed.

Members of Uddin’s family, who travelled from Bangladesh to attend the three-week trial, hugged and cried after Syeedy was led away.

The murder has alarmed counter-terrorism experts, who fear that homegrown extremists are increasingly targeting fellow Muslims whom they view as heretics.

Uddin was bludgeoned to death with a hammer on his way home from a mosque in Rochdale on 18 February. He was attacked with repeated, forceful blows from behind after having been under surveillance by his killers for months, the trial at Manchester crown court was told.

Jalal Uddin was an imam at the mosque Syeedy attended.

Jalal Uddin was an imam at the mosque Syeedy attended. Photograph: Greater Manchester Police/PA

The judge, Sir David Maddison, said he did not believe the intention was to kill Uddin, but to “cause extremely grave and permanently disabling harm” so that he was unable to practise Islamic healing ever again.

Syeedy played an “absolutely integral” part in the murder, the judge said, describing the killing as a “carefully planned”, “premeditated” attack on a vulnerable victim.

Describing Uddin as “gentle, well-respected”, the judge said the “brutal” killing was a hate crime but did not match the sentencing threshold of a religiously motivated murder, which carries a minimum term of 30 years’ imprisonment.

“It seems to me that this was a case of two members of the Muslim faith killing another member of the Muslim faith solely because they disapproved of a particular practice carried out by that person,” Maddison said.

Jurors were told that Syeedy and his alleged accomplice, Mohammed Abdul Kadir, stalked Uddin around the streets of Rochdale before Kadir launched the attack on the older man in a children’s playground.

Syeedy, the getaway driver, denied knowing about the murder plot.

Kadir, a former John Lewis call centre worker, was not on trial because he fled to Istanbul three days after the murder. Counter-terrorism detectives are hunting for the 24-year-old, but believe he may have crossed the border into Syria to join Isis.

Syeedy and Kadir, from Rochdale and Oldham respectively, developed a hatred of Uddin last summer after discovering he practised a form of Islamic healing called taweez, the court was told.

Uddin moved to Rochdale 15 years ago from Bangladesh. The former imam was well known in the area for his use of healing amulets to ward off ill health and protect people from evil spirits.

Uddin’s friends warned that the killing had left an open wound in Rochdale’s Muslim community, as his family paid tribute to a “greatly respected man, a caring and loving soul”.

Uddin’s relatives said they had been “left empty” by his murder in a statement released by police.

“We came to court to seek answers from Mohammed Hassan Syeedy. He has shown no remorse or sympathy towards Jalal or his family, as indicative of his attitude and demeanour throughout the last four weeks,” the family said.

“Although Jalal was a Muslim who peacefully practised his faith, he had a love and respect for all religions, cultures and creeds, and the fact that he was murdered by someone inspired by Isil shows the true nature and barbarity of this organisation and those who serve it.”

In a victim impact statement read to court, one of Uddin’s seven children, Sala al-Arif, said his father told him two days before he died that he planned later this year to return to Bangladesh for the first time in 15 years to see his wife, children and grandchildren.

Instead, he said, his relatives have been left with an “everlasting void”. Al-Arif said that when he visited his father’s body in the mortuary he was “denied the most basic human right” to kiss his father’s face because of the horrific injuries he had suffered.

Describing his father as a “devout pacifist who showed nothing but love to anyone he came across,” Al-Arif said Uddin’s relatives had been left without answers as to why he was targeted: “I cannot begin to understand why anyone would want to murder him. This is the ultimate question for me and my family: why was my father murdered?”

Jurors were told that Syeedy and Kadir, who were not known to the police, believed Uddin’s use of taweez was black magic and that he deserved to be killed.

Uddin, known as the Qari Saab by his followers for his deep understanding of the Qur’an, was mocked as a magician by Syeedy and his friends, who nicknamed him Voldemort after the evil wizard in the Harry Potter books.

Six months before the murder, Syeedy and his friends destroyed Uddin’s notes and books on taweez after stealing them from their mosque. They then started monitoring Uddin’s whereabouts, taking covert pictures of him in the street, as they plotted to have the community leader deported by immigration services.

But that plan was abandoned after a photograph emerged of Uddin with the Rochdale MP, Simon Danczuk, outside the Jalalia mosque in December last year. One of Syeedy’s friends shared the photograph and commented: “Oh crap … voldermort nvr gna b busted by immigration now.”

When detectives arrested Syeedy five days after the murder, they found gruesome footage on his phone of Uddin dying, along with reams of Isis propaganda.

Jurors were shown photographs of Syeedy holding an Isis-style flag outside the Jalalia mosque. In another image, he wore a stabproof vest outside the mosque.

Other photographs showed Syeedy and two people holding a jihadi flag over a road sign in Rochdale that had been altered to read: “War zone.”

Detectives also found a black, Isis-style baseball cap, arm patches and a flag in Syeedy’s home, where he lived with his mother, younger sister and younger brother.

Paul Greaney QC, prosecuting, told the court that Syeedy had been drawn in by Isis propaganda surrounding the Syrian war over a period of three years.

In 2013, he travelled to Syria on a medical aid convoy at around the same time that Alan Henning, the murdered Salford taxi driver, went on a similar “Rochdale to Syria” trip, the trial heard.

Syeedy told the court he was friendly with Henning and was “disgusted” when he heard that the 47-year-old had been beheaded by an Isis militant who was revealed last year to be Londoner Mohammed Emwazi.

Det Ch Supt Tony Mole, head of the north-west counterterrorism unit, said: “This has been a long and delicate investigation, with detectives working meticulously to establish exactly what happened to Jalal and I want to thank them for their work.

“I would also like to thank the local community for the patience and support they have shown us. Their assistance and understanding has been vital and they have been a credit to Rochdale.

“I hope the outcome here today will bring some closure to Jalal’s family and help them continue to rebuild their lives following their tragic loss.”

One of Uddin’s friends, Mohammed Shafiq, chief executive of the Ramadhan Foundation charity, urged the Rochdale community to come together or “we may see such tragedies again”.

He added: “It is beyond doubt that these young minds are being brainwashed and groomed online, they have no engagement or involvement with any of the mosques and these institutions are failing young Muslims.

Giving evidence, Syeedy denied knowing anything about Uddin’s murder, but admitted he was with Kadir before and after the attack.

He said he never suspected that his friend, whom he knew was an Isis sympathiser, had killed Uddin, even though the former imam was found dead moments after Kadir left the playground.

Syeedy, an electrical engineering student who dropped out of university, denied being an Isis supporter and described the group’s actions as “completely wrong”.

He told jurors he did not agree with Uddin’s use of taweez because it was dangerous to meddle with the supernatural world, but that it was up to God to deliver the punishment. “This is my country, I abide by the laws,” he told the jury.