Two incidents involving U.S. forces in the predominantly Shia southern Iraq over the past week appear to demonstrate the growing complexities and dangers of the country’s civil conflict.

Sunday’s day-long battle near Najaf, in which two U.S. pilots were killed when their military helicopter was shot down, was first reported as an attack by Sunni insurgents and “foreign fighters” on the holy city and the tens of thousands of Shia pilgrims who are converging there for Ashura. But later reports identified the heavily armed and highly organized assailants as members of the Army of Heaven, an obscure Shia sect that believes the killing of Najaf’s senior ayatollahs, including Grand Ayatollah Ali al-Sistani, will hasten the return of the Mahdi (messiah or leader).

The Iraqi government claimed that 200 Army of Heaven members were killed, including the group’s leader, and another 120 captured after some 15 hours of fighting, which reportedly came as a major surprise to U.S. officials despite the large number of Army of Heaven fighters involved and the firepower—a heavy machine gun downed the U.S. helicopter, according to the Pentagon—at their disposal.

“If they had succeeded in their plans, the political consequences would have been catastrophic,” said one Washington official who noted that this was by far the heaviest fighting in the south since the 2004 insurrection by Moqtada al-Sadr’s Mahdi Army. “The fact that we had no forewarning when so many people were involved shows how limited our intelligence is not just in al-Anbar Province [the stronghold of the Sunni insurgency] and Baghdad, but in the south, too. We really don’t know much about what is going on there.”

U.S.-led coalition forces formally returned control over security to Iraqi forces just last month.

Meanwhile, officials here are still trying to figure out who was behind the January 20 surprise attack on a U.S. security team that was meeting with its Iraqi counterpart in the regional government offices in Karbala, some 50 miles north of Najaf. After initially reporting that five U.S. soldiers were killed defending the compound, the Pentagon reported late last week that only one was killed in the initial attack, while the other four were abducted and later shot execution-style about 30 miles to the east where their bodies were found.

The Associated Press (AP) reported that as many as a dozen attackers traveled in the kind of convoy of SUVs frequently used by U.S. officials in Iraq. They wore U.S. combat fatigues, and at least several of them spoke English, according to Iraqi soldiers who waved them through a checkpoint on the outskirts of Karbala. The SUVs and uniforms apparently involved in the attack were later found abandoned with the bodies in Mahawil in Babil Province after Iraqi guards at one checkpoint gave chase.

“The precision of the attack, the equipment used, and the possible use of explosives to destroy the military vehicles in the compound suggests that the attack was well rehearsed prior to execution,” a military spokesman in Baghdad told the AP.

While the U.S. military has announced the arrest of four suspects in the attack, which was unprecedented in its sophistication, no further information has been released, fueling speculation both within the government and among independent analysts as to who was behind it.

Juan Cole, an expert at the University of Michigan, said he believed the attackers’ final destination in Mahawil, a mixed Sunni-Shia city in an area of intense Sunni guerrilla activity, suggests that Sunni insurgents were responsible and that the raid “was aimed at harming security arrangements” for the Ashura pilgrimage that ended this week in Karbala, where tens of thousands will commemorate the martyrdom of the Prophet Mohammed’s grandson, al-Hussein.

But the precision of the attack—only U.S. soldiers were singled out—also points to an inside job, according to Cole. “How would there have been Sunni Arab guerrilla sympathizers at this police and army meeting at Shia Karbala?” he asked, suggesting that perhaps “mixed units” were involved.

Others see the attack as part of an increasingly dangerous game of “tit-for-tat” between U.S. and Iranian-aligned forces. Ray Close, a retired top Middle East analyst at the Central Intelligence Agency (CIA), suggested that the attack and the abductions may have been retaliation for two recent raids in which the U.S. military seized and abducted Iranian officials in Iraq—the first in Baghdad on December 21, the second in the Kurdish city of Irbil on January 10.

After protests by the Iraqi central government, as well as by Tehran, the Iranians arrested in the first raid were released and deported home. The fact that the raid took place at the offices of the leader of the Supreme Council for the Islamic Revolution in Iraq, Abdul Aziz al-Hakim, who had just returned from Washington where he was treated as an honored guest at the White House by President George W. Bush, naturally added to consternation over the incident.

The five Iranians seized in the second raid, which also elicited protests from both Baghdad (notably President Jalal Talabani) and the local Kurdish authorities, have not yet been released, although officials in Tehran hinted Monday that they had received a message from Washington regarding a resolution of the case.

Both raids came amid escalating charges by Bush, as well as other senior U.S. officials, that Iran is providing “material support for attacks on American troops” and threats to, in Bush’s words, “seek out and destroy the networks” that are allegedly doing so. The U.S. Embassy in Baghdad is expected to elaborate on U.S. charges based in part on materials seized during the two raids.

In a widely circulated memo, Close cites a “very knowledgeable friend,” a recently retired analyst from the Defense Intelligence Agency, who noted Washington has had contingency plans for “specific paramilitary actions against Iranian personnel inside Iraq in case Iranian support for the insurgency became a significant problem,” and that such actions would likely provoke retaliation.

“My friend looks at the recent incident in Karbala as very probably an Iranian operation carried out in retaliation against the recent seizures of Iranian operatives by the U.S. in Baghdad and Irbil,” Close wrote. “He says that the sophistication of the Karbala operation seems far beyond the capabilities of the Iraqi insurgents and indicates the high probability of Iranian planning and execution.

“We need to watch carefully now to see if the ‘tit-for-tat’ game between the United States and Iran continues to escalate, and if in the end it proves to be a game that we might have been wiser to avoid or to minimize as much as possible,” Close wrote.

At least one thing is relatively clear: Iran was unlikely to have been behind Sunday’s attack on Najaf. Adherents to the Army of Heaven are reported to be violently anti-Iran; they burned down Iranian consulates in Basra and Karbala last year.

Jim Lobe is the Washington bureau chief of the Inter Press Service and a contributor to Right Web (rightweb.irc-online.org).