A man reads today's paper in Nairobi on Sept. 25, 2013 after Kenyan forces took back control of Westgate mall following a deadly four-day siege that left more than 60 people dead.

Kenyan authorities are questioning four mobile companies over concerns about unregistered phone lines that were possibly used by Islamists when they attacked the Westgate shopping mall in Nairobi several weeks ago, according to BBC News.

The move came amid concerns that some unregistered SIM cards were sold to the perpetrators of the attack, potentially allowing them to communicate without being detected, said the Associate Press.

Kenya's criminal investigations department director, Ndegwa Muhoro, told the AP on Tuesday that the four chief executives had been called in for questioning.

The bosses were told they could be arrested if they didn't cooperate, said the BBC.

Their companies — Safaricom, Telekom, Yu and Airtel — have denied activating any unregistered lines, said the BBC. All SIMS must be registered under Kenyan law.

But Kenyan communication head Francis Wangusi told the BBC that "unregistered SIM cards are still on their networks and police have proven that."

He also told the BBC there are 1.6 million unregistered SIM cards in the country.

The attack on Nairobi's Westgate Mall killed at least 67 people.

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