After three days of measurable progress, American forces trying to take full control of Fallujah are confronting an insurgent force that has renewed energy.
And as American and Iraqi forces spread their grip across the city, the constant skirmishes of close urban combat and burst-in searches door-to-door are revealing more about insurgent tactics, including sleeper cells.
Thursday night, forces braced for a significant counteroffensive by Iraqi insurgents - an effort coinciding with the "Night of Power," an annual Islamic holy day marked by intense spiritual devotion, which is said to cleanse sins and determine destiny.
Loudspeakers from at least one mosque began what US Marine officers said was a "revving up" of militants by chants that resembled the "martyr's last rites."
"We expect an increase of suicide attacks, by cars, motorcycles, and people wearing explosive vests," said Lt. Col. Michael Ramos, commander of the 1st Battalion 3rd Marines, from Dallas.
"It's going to get a lot worse tonight," Capt. Gil Juarez, commander of the Light Armored Reconnaissance (LAR) company told his platoon chiefs, as marines loaded their vehicles at dusk with extra ammunition.
"I think there's going to be a big fight tonight, so get your heads ready to get back in the game," said Captain Juarez, from San Diego, Calif. "These [insurgents] are pushing the offensive right now. We've got to get ready."
Despite a day of steady US tank and artillery fire that rumbled across Fallujah like a thunderstorm ready to pour, insurgents began their work.
One armored unit was ambushed in the south center of the city by militants who struck with rocket-propelled grenades. Separately, another vehicle was hit with gunfire, wounding a marine.
Those involved in the ambush said a trap had been laid, and that the area was marked with earth berms in defensive posture, and metal-box firing positions. Shortly after the firefight, US-fired artillery rounds crashed into the area.
"They have been working on it, an L-shaped ambush," said one corporal, whose face was blackened by smoke from the attack. "It looks like something out of Mad Maxx."
"We walked right into a hornet's nest today," said a sergeant with the worn look of a survivor. Their names and units involved could not be released, in line with military rules that prevent such details until the wounded's next of kin have been notified. "They were probing us and fired six RPGs before we went for it. They lassoed us right in."
As American and Iraqi forces have spread their grip across Fallujah, the constant skirmishes of close urban combat and burst-in, door-to-door searches are revealing more and more about insurgent tactics.
In the course of locating seven weapons caches in a single block around a mosque in northeast Fallujah, an Iraqi platoon Wednesday found a suitcase full of vials labeled "Sarin," a deadly nerve agent.
While further analysis determined that the find was probably part of a Soviet test kit with samples, its discovery in a room with mortar shells appeared to indicate an intent to weaponize the material.
On the eve of the US-Iraqi assault on Fallujah, insurgent leaders in the city promised a massive counterattack.
Until late Thursday, resistance in Fallujah had been piecemeal, with individual rocket, mortar, and rifle teams making surprise attacks. US heavy artillery, tank guns, and airstrikes have waged steady barrages, paving the way for marine infantry advances.
US military leaders have deemed the effort in Fallujah so far as a success. In three days of fighting, coalition units have swept across more than half the city, sustaining relatively few casualties.
But Thursday night, casualties appeared to mount. Coalition forces have been targeted from mosques. They have uncovered unarmed sleeper cells that they believe have been seeded throughout the city and primed to strike after the initial assault.
Insurgents continued a wave of violence elsewhere. A car bomb ripped through a crowded Baghdad commercial street, killing 17 people, police said. In the north, guerrillas overwhelmed several police stations in Mosul and battled US troops.
Mosques were used by militants when marines first attempted to invade the city last April. They were sometimes targeted by US forces, adding to the international outcry that grew at the time about civilian casualties.
This time, Iraqi nationalists and Islamic militants loyal to the network of Al Qaeda affiliate Abu Musab al-Zarqawi, may have been depending on mosques as staging areas, US officers say.
"Almost every single mosque we've come through has been used for weapons storage and insurgent military training," says Lt. Col. Ramos.
Marines have shot at the speakers of minarets, which are normally used for the Muslim call to prayer, though in recent days they've served as a literal call to arms.
Before the assault began, US intelligence officers warned of unarmed Iraqis wearing dishdashas (traditional long gowns worn by men) moving to US lines and reporting back to guerrilla cells.
Instead, marines have found that small groups of unarmed men, claiming to have stayed behind to prevent looting of their house, may in fact be sleeper cells, waiting for orders to link up with prepositioned weapons and attack.
One example Wednesday was a group of four men, found in their house by the LAR Raider Platoon during a search. They said they had recently been caught by the mujahideen, or holy warriors of the resistance, and been tortured.
Later that day and several blocks away, Raider Scouts searching other buildings found four more men. They also said they stayed behind to guards their houses, and that they had been tortured.
But further questioning found that there were no signs of torture - militants in Fallujah typically kill suspected traitors - and that the men's claimed identities did not hold up to investigation.
"It was well rehearsed," said Lt. Michael Aubry from Arlington Heights, Ill. "The first time didn't look suspicious, but the second time ... it did."
"There are sleeper cells all over the place," says Juarez. "They are either going to start coming out of their holes and attack us, or [they] will leave."
• Material from wire service reports was used in this article.