How did the Dallas gunman get hold of an armoured van described as a 'Zombie apocalypse assault vehicle'?

The Facebook advert for the van described it as capable of 'drive-by mow-downs'

The armoured van was set ablaze after the Dallas stand-off (WFAA)

As a barrage of bullets sprayed out of a dark blue van used to stage a brazen assault on Dallas police headquarters early on Saturday morning, even witnesses recognized this was no ordinary form of transportation.

The beast of a van — sold just days ago — eerily lived up to how it was advertised on Facebook, on which it was touted as a "Zombie apocalypse assault vehicle" with "gun ports" capable of "drive-by mow-downs" and full armour and bulletproof windows "just in case someone might try to take this bad boy from you."

Investigators suspect the van might have belonged to a sheriff's office in Georgia before it was decommissioned and sold off. It was most recently sold by Jenco Sales out of Georgia. The business owner could not be reached for comment.

Police block the intersection of Dowdy Ferry Rd and Interstate 45 during the stand-off (Brandon Wade/ AP)

Read more: Dallas shooting as it happened

The Ford van was sold on eBay for $8,250. At one point — even when there were no competing offers — that bidder increased the offer from $6,000 to $7,000, and then to $8,000.

The van held up in what Dallas police Chief David Brown described as the "helter skelter" shootout at police headquarters, but he noted the vehicle was not invincible.

The van as pictured on its eBay listing (eBay)

"Our Swat snipers shot out the engine block of the suspect vehicle with our .50-caliber sniper rifle," he said.

The van was stopped and later set ablaze, ending the public safety nightmare. But law enforcement sources say there are more of these vehicles still for sale online.

Copyright WFAA-TV Dallas-Fort Worth/USA Today