On Wednesday, Defense Secretary James Mattis confirmed to reporters that the United States and its allies are “still assessing the intelligence” about the alleged chemical weapons attack on Syrian civilians this weekend.

In response to a question about whether U.S. military forces are “ready right now to conduct a retaliatory strike if ordered,” Mattis added: "We stand ready to provide military options if they are appropriate as the President determines.”

Mattis’s comments followed President Trump’s tweet this morning promising that missiles “will be coming” to Syria soon:

Russia vows to shoot down any and all missiles fired at Syria. Get ready Russia, because they will be coming, nice and new and “smart!” You shouldn’t be partners with a Gas Killing Animal who kills his people and enjoys it!

— Donald J. Trump (@realDonaldTrump) April 11, 2018

In spite of Trump’s above certainty about Assad’s culpability in the chemical attack, on Monday, the president said that American intelligence had not been able to confirm who had carried out the attack. He also expressed frustration that the area was not immediately made open to international investigators [emphasis mine]:

.@POTUS Trump condemns the heinous attack on innocent Syrians with banned chemical weapons. #Syria pic.twitter.com/qiEahlL3Ah

— Department of State (@StateDept) April 9, 2018

I’d like to begin by condemning the heinous attack on innocent Syrians with banned chemical weapons. It was an atrocious attack. It was horrible. You don’t see things like that, as bad as the news is around the world, you just don’t see those images. We are studying that situation extremely closely. We are meeting with our military and everybody else, and we’ll be making some major decisions over the next 24 to 48 hours.

We are very concerned when a thing like that can happen. This is about humanity. We’re talking about humanity. And it can’t be allowed to happen. So, we’ll be looking at that barbaric act and studying what’s going on. We’re trying to get people in there. As you know, it’s been surrounded, so it’s very hard to get people in, because, not only has it been hit, it’s been surrounded. And, if they’re innocent, why aren’t they allowing people to go in and prove -- ‘cause as you know, they’re claiming they didn’t make the attack.

So, if it’s Russia, if it’s Syria, if it’s Iran, if it’s all of ‘em together, we’ll figure it out.

For the moment, Western governments attributing blame for the chemical attack on Assad’s government are, based on mainstream reports from outlets like the BBC, relying almost entirely on Islamist rebel groups and the activists and NGOs that operate within their territory for information [emphasis mine]:

Syrian opposition activists, rescue workers and medics say more than 40 people were killed on Saturday in a suspected chemical attack on Douma, the last rebel-held town in the Eastern Ghouta region.

They allege that bombs filled with toxic chemicals were dropped by Syrian government forces. The government says the attack was fabricated.

(…)

In March, troops split the region into three pockets - the largest of which was around Douma, home to between 80,000 and 150,000 people. Facing defeat, rebel groups in the other two pockets agreed to be evacuated to northern Syria.

But the group controlling Douma, Jaysh al-Islam, continued to hold out.

(…)

Activists from the Violations Documentation Center (VDC), which records alleged violations of international law in Syria, reported two separate incidents of bombs believed to contain toxic substances being dropped by the Syrian Air Force.

(…)

At 19:45, more than 500 patients - most of them women and children - were brought to medical facilities with symptoms indicative of exposure to a chemical agent, according to the Syria Civil Defence and the Syrian American Medical Society (SAMS), a relief organisation that supports hospitals.

(…)

The Union of Medical Care and Relief Organizations (UOSSM), which supports hospitals in rebel-held Syria, also said it received reports of two incidents [of chemical attacks].

Jaysh al-Islam (JAI, “Army of Islam”), the group that controls the area alleged to have been attacked by Assad, is an Islamist group that has acted as a rival of both ISIS and the al-Qaeda-affiliated al-Nusra Front, according to Middle East Eye [emphasis mine]:

JAI formed after a merger involving around 60 groups, including Liwa al-Islam, and is itself one of the main components of the Islamic Front – a group of Gulf-backed fighting groups – and are thought to be second only to Ahrar al-Sham in terms of power and numbers.

The Islamic Front issued a charter in 2013 (prior to Jaish al-Islam's joining) that laid its principles for the creation of an Islamic-rooted society in which Islam would be the “religion of the state, and it is the principal and only source of legislation.”

(…)

“O mujihideen brothers! We will leave these fields in which we finished our course and preparation and we will continue with preparing to wage jihad,” says the group's leader, Zahran Alloush, speaking to the recruits from a podium.

“Today the world is conspiring against us. And we have no one but Allah, an excellent protector and helper is he!”

Of course, the fact that the allegations of the chemical attack are coming from an Islamist-controlled area with a vested interest in bringing international scorn down on Assad does not mean that they are false. After all, it was Nazi Germany that uncovered the Soviet mass murder of Polish officers in the Katyn Forest back in 1943. However, while it may not be impossible for a warring side to use an actual massacre or war crime committed by their enemies to paint them in a bad light, there is also no guarantee that such accusations are accurate.

In the past, President Trump was much more skeptical of the idea that the United States should support Islamist rebels and believe their claims without question:

We should stay the hell out of Syria, the "rebels" are just as bad as the current regime. WHAT WILL WE GET FOR OUR LIVES AND $ BILLIONS?ZERO

— Donald J. Trump (@realDonaldTrump) June 16, 2013

Remember, all these ‘freedom fighters’ in Syria want to fly planes into our buildings.

— Donald J. Trump (@realDonaldTrump) August 28, 2013

Many of the Syrian rebels are radical jihadi Islamists who are murdering Christians. Why would we ever fight with them?

— Donald J. Trump (@realDonaldTrump) September 6, 2013

That skepticism sure appears to be gone now.

https://townhall.com/tipsheet/chrisreeves/2018/04/11/secretary-mattis-us-government-not-sure-who-carried-out-syria-chemical-attack-n2470044