The North Korean nuclear test wasn’t exactly a shock-Kim Jong Un’s cronies said they had developed an H-Bomb. But U.S. officials were still on the back foot when the bomb went off.

The U.S. military insisted Wednesday that it was was not surprised “strategically” by North Korea’s test of a nuclear device. But Pentagon and intelligence officials admitted that they were caught off guard by the specific timing of the detonation.

One U.S. defense official told The Daily Beast that the Hermit Kingdom’s test was a “tactical surprise.” But given North Korea’s recent claims that it had developed a powerful hydrogen bomb, officials were expecting some kind of test. (Experts had dismissed the country’s claim last month that it had developed a true “H-bomb,” and the White House on Wednesday said that initial data from monitoring systems in Asia were “not consistent” with what one would expect to see from the powerful device.)

U.S. intelligence officials had also been expecting a potential test from North Korea in recent weeks, though it’s not clear that they had predicted the exact timing, either. “There had been reports on this for months,” one such official told The Daily Beast. But there was surprise “that they [the North Koreans] had picked this particular day.”

“There was a lot of anxiety about a possible test last month among intelligence analysts and weapons experts,” Jeffrey Lewis, the director of the East Asia Nonproliferation Program at the James Martin Center for Nonproliferation Studies at the Monterey Institute of International Studies, told The Daily Beast.

There were some signals that the U.S. military wasn’t fully prepared for an imminent test. One of its two WC-135 Constant Phoenix, a specialized nuclear-detection aircraft sometimes referred to as the “the sniffer,” wasn’t deployed from Offutt Air Force Base in Nebraska to Caldina, Japan, until Wednesday. That was hours after the reported test.

The WC-135 collects air samples and can identify radioactive materials, gases, and debris that may be released by a nuclear explosion.

It’s not clear why the military didn’t deploy the aircraft closer to the region sooner if officials were anticipating a nuclear test. North Korea has been under United Nations sanctions since 2006, when it first conducted nuclear tests. Since then, and following almost every subsequent test, the U.S. military has deployed the WC-135.

The defense official told the Daily Beast that the ground-based sensors in Asia collected air samples before and after North Korea announced it had successfully created a hydrogen bomb. Seismic monitors around the globe began reporting evidence of a possible nuclear detonation minutes after the test. And NBC News reported the U.S. military also launched drones over North Korea to collect air samples.

In addition, the American intelligence community maintains a constellation of spy satellites whose primary mission is to collect measurement and signature intelligence, or “MASINT”-tell-tale electromagnetic signs of a nuclear blast, among other things. At least one of these satellite arrays maintains 24-hour coverage of areas including North Korea.

So far, there has been no publicly revealed evidence that North Korea tested a hydrogen bomb.

“The U.S. government judges North Korea to have conducted a nuclear test yesterday,” State Department spokesman John Kirby told reporters Wednesday.

The test itself appeared not to raise significant concerns in military and intelligence circles that North Korea now possesses the devastating weapon. A hydrogen bomb has a potential yield thousands of times larger than traditional nuclear weapons.

In the halls of the Pentagon, there was hardly a worry about the safety the 28,000 U.S. troops deployed nearby in South Korea-and whether they had been exposed to dangerous levels of radiation.

But it will take several days to determine precisely what exactly North Korea detonated, experts said. The “sniffer” aircraft will play a key role in that effort.

In 2010, the Congressional Research Service said it could help testing efforts to have more than one WC-135 available. (Of the two in the fleet, one is usually in a maintenance depot. Both aircraft are also more than 50 years old.)

“Given the prospect that several nations, over a vast geographic area, might conduct nuclear tests,” the CRS reported, it was important to be able to collect samples “as soon as possible to narrow the region where the test occurred…” The quality and quantity of the samples diminishes over time.

The CRS also said it might be cheaper to use long-range drones, such as the Reaper and Global Hawk, instead of the large sniffer plane. It’s not clear what type of drone may have been deployed in the most recent test. (In the past, the Air Force has also use the stealthy, secretive RQ-170 drone to monitor North Korea.) But the drones mentioned in the report can carry sensors that would collect air samples-samples that would be crucial to understanding Pyongyang’s blast.