Northern Nigeria's riot police training academy has been overrun by Boko Haram Islamist militants, a witness in Borno state has told the BBC.
Shots were heard after the militants arrived in three armoured vehicles and on dozens of motorcycles, he said.
A police spokesman confirmed the attack while a senior security source said it had not been possible to communicate with the academy since Wednesday.
The Liman Kara college is near Gwoza town, seized by Boko Haram this month.
Thousands have been killed across north-eastern Nigeria since Boko Haram launched its violent campaign for an Islamic state in 2009.
The militants have stepped up their attacks after being pushed out of their bases in Maiduguri, the capital of Borno state, and have been targeting towns and villages in deadly raids.
In recent weeks, the militants have been moving from their rural camps and taking over substantial towns.
The militants have been in control of Gwoza, which had a population of about 50,000, since the beginning of August.
They apparently retreated about 100km (62 miles) to Gwoza after losing control of Damboa - both large towns in Borno state.
But attempts by the security forces to retake Gwoza have failed - and a group of about 40 soldiers is now refusing to fight, saying they are too poorly equipped to take on the heavily armed insurgents.
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