The Iraqi city of Fallujah was thought by many to be empty.
Islamic State of Iraq and the Levant (Isil) fighters were reported to have all fled, but the elite troops who had been fighting in the city for a month knew better than to take their enemy lightly.
The advance line of army troops sent in yesterday (Sunday) to finish the job were caught off guard.
As they moved between bombed-out buildings in the former Isil stronghold, they came under attack as a sniper let off a round from his hiding place.
Several hundred jihadists are still holed up in Fallujah, officers confirmed, and they are mounting a desperate defence.
Iraqi forces pushed deeper into Fallujah yesterday, slowly tightening the noose around the fanatical core of Isil fighters determined to fight to the end.
Spearheaded by elite counterterrorism units known as the Golden Division, troops inched forward, slowed by sniper fire, suicide attacks and an urban landscape rigged with explosives.
Iraqi prime minister Haider Abadi declared on Friday that the battle won after they raised the Iraqi flag over the central government compound. But the reality on the ground is very different.
"Baghdad is our capital of the country, but for Daesh it is Fallujah," Imad Kareem, a fighter in one police unit, told The Daily Telegraph.
"All the other places you have heard about, like Tikrit or Ramadi, they have been easy, but this fight is hard."
In response, the jihadists are pounded mercilessly.
The city is a devastated shell after the vast majority of civilians -
some 60,000 of them have been trapped in Fallujah since the fighting began - managed to escape when Isil withdrew from key checkpoints on Thursday to concentrate their fighters in a last- ditch defence.
Unconstrained by fears of civilian casualties, the army is now bombarding the city without cease.
From beyond the ruined outskirts, batteries of 155mm howitzers
and Katyusha rocket launchers pour fire into the centre, where the thud of exploding artillery shells mixes into the constant crackle of gunfire. Occasionally, a rocket screeches overhead.
The Golden Division entered the city from the south, and yesterday continued to fight its way north into Fallujah's Old Town.
In the adjacent Analous neighbourhood, police units provided fire support from sniping positions in abandoned houses.
On a rooftop in Baghdad Street, which separates the Old Town from Andalus, a group of policemen looked intently at the screen of a tablet.
They had launched a drone above the areas still held by Isil, and
now monitored their enemy's movement.
Two mortars tubes had been placed on the roof, and were put to use soon after the drone returned.