Morocco faces questions over factory drownings
"The victims were trapped with no way to get out" and drowned, senior fire and rescue service official Abderrahim Kabajj told Morocco's 2M television station.
An official statement on the incident referred to an "illegal textiles operation".
But a police source told AFP the status of the workshop, located in a residential area in the city's southeast, was "yet to be confirmed", adding the owner had not been questioned as he was still in intensive care.
Morocco's informal sector plays a key role in the economy, with over half of the country's textile and leather production coming from unregulated operations, according to Morocco's employers' association.
Many fail to meet official safety standards, it says.
The workshop owner's sister took to YouTube to defend her brother, saying the business was legal.
"He pays the taxes and the employees are declared to the CNSS," she said, referring to Morocco's social security system.
She alleged the state was trying to shirk its responsibilities for the flooding.
A representative of the country's textile industry association also said that "the company has business registration and is completely in order".
Requesting anonymity, he said there were "hundreds" of such workshops in Tangiers, "keeping alive thousands of families".
"I can't imagine they would all be illegal as they are working for big international brands," he said, without specifying which.
He pointed to "failing infrastructure" and called for accountability.
The deaths have caused an outcry in the North African country.
The L'Economiste newspaper in an editorial denounced "factories of poverty" and said the tragedy had refocused attention on labour rights.
Mohamed Benaissa, head of Morocco's Northern Observatory for Human Rights, said the workshop "has existed for 10 years", pointing to a high voltage power connection "requiring official certification".
It was not the only such factory in Tangiers, he told AFP previously, adding that his association had observed "many other workshops in the basements of houses without respect for safety regulations, even during the coronavirus pandemic".
In a statement on Tuesday, the CDT union denounced the proliferation of "illegal" workshops in Tangiers, the "exploitation of workers" and "the absence of appropriate working conditions".
A recent central bank report said the informal sector accounted for a third of Morocco's gross domestic product.
Meanwhile, a town planning official told media the building was part of a housing project constructed near a river at flood risk.
Abdelaziz Janati, a lawyer and human rights activist, told AFP the status of such workshops was often problematic.
"If they (workshops) are unauthorised, it's considered a crime," he said.
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"But it's also a crime if they are authorised, because they don't respect the conditions necessary for industrial production and they're not in an industrial zone.”
Agencies contributed to this report.