The first tip came from a casual conversation with a source back in May: Health and Human Services Secretary Tom Price was using private jets for routine travel, possibly in violation of federal travel rules that allowed such flights only when commercial options weren’t available.

But it was a tip and little else—no times, no names of charter services and not even a schedule from a notoriously secretive Cabinet secretary.

So we embarked on a months-long effort to win the trust of sources, both in and outside of HHS, who were in a position to know about the secretary’s travel. This required numerous meetings and phone calls, sometimes after hours, seeking to confirm what the original source acknowledged was just secondhand information. Neither of us had ever reported a story of this difficulty before.

Price’s lack of transparency made our job harder. In the initial months following his confirmation on February 10, he hadn’t made his schedule public, unlike past secretaries. Bare-bones reports of events outside Washington usually were posted on the agency’s website after they happened. But there were few places and times—not enough information to take either to charter services, sources inside HHS or anyone else in a position to know about the flights. And we faced another problem: Because the planes Price was taking were private, their arrivals and departures weren’t recorded on public databases.

That meant we had to re-create Price’s schedule from scratch if we were to have any hope of matching his trips to chartered flights. We reviewed the HHS summaries of Price’s meetings. We scoured news sites for reports of Price speeches outside Washington. We obsessively tracked his appearances on social media. Putting all this information together, we built a database of Price’s trips.

That task became somewhat easier in July, when Price’s public relations staff began sending out notices to reporters of upcoming trips—a common practice in previous administrations—but it wasn’t until late August that the reports of the secretary’s travel plans became more regular.

This created an opportunity to stake out the airport and watch Price get on and off the plane, the surest way to determine that he was using charter aircraft on dates and times when commercial flights were also available. But which airport was Price traveling from?

There were several missed opportunities, when we either went to the wrong airport or the right airport at the wrong time. Then, in September, we received an official notice from HHS that Price would be traveling to the Philadelphia area to meet with people affected by addiction to opioid painkillers. By the morning of September 15 — when Price took a private jet to travel from Washington to Philadelphia, a distance of roughly 125 miles — we had figured out that Price was using the private jet terminal at Dulles International Airport, 28 miles outside of Washington.

That Friday morning, we camped out at the Dulles charter terminal. A little after 8 a.m., we saw two SUVs and a police escort roll onto the tarmac, as the cars discharged passengers who then boarded a distinctive 30-seat charter plane with a golden belly. By 8:30 a.m., we watched the charter jet take off, heading north, and tracked it to Philadelphia using FlightAware, an airline tracking website.

It was strong evidence, but it wasn’t enough. We hadn’t seen the faces of Price or the other passengers — including White House counselor Kellyanne Conway, who went on that opioid trip. Using social media and help from sources, we kept tabs on the movements of Price and Conway in Philadelphia, tracing when they wrapped up the event and headed to the airport.

By the time Price boarded the private jet at Philadelphia International Airport for the return trip to Dulles in early afternoon, we had figured out how to get the best view of the Dulles tarmac: Rachana drove by the charter jet terminal, timing her approach so she could best see the jet field, while Dan stood in the airport’s main terminal, tracking the plane through his phone and counting down its arrival to Rachana, before hustling out to a patch of concrete that offered a view of the tarmac, too.

It was enough. We saw the distinctive golden plane taxi around to the private-jet terminal. We saw a man with Price’s build and shock of white hair walking across the tarmac. We saw the SUVs and police escort come out to greet him and the other passengers before speeding them away.

After that, we pieced together his itinerary for the entire week by calling sources, looking for photos of events on social media and compiling HHS notices for his travel plans. We even called charter plane companies—including the one that Price had used—to figure out how much the trips cost. And we purchased flight data that offered the precise times for departures and arrivals of flights to validate what sources had said was Price’s schedule. That also enabled us to compare Price’s actual departures and arrivals with commercial flight options, train travel and even the cost of traveling by SUV. This was especially important: We wanted to compare the cost of Price’s jets with that of commercial transportation, revealing a potential waste of taxpayer dollars.

While there was evidence that Price had been using charters for months, our first story stuck to what we had established beyond any doubt: The round-trip to Philadelphia, as well as three other flights in the previous two days.

On Monday, September 18, we sent our questions to HHS and repeatedly asked for a meeting or phone call with Price, but were rebuffed. After more than 24 hours, HHS issued a terse statement that, yes, the secretary used charter flights, but mostly for public health emergencies. The statement asserted that there was no violation of federal regulations, but offered no further explanation.

The publication of our first story—headlined “Price’s private-jet travel breaks precedent”—shook loose new sources of information and gave fresh impetus for us to go back to our earlier efforts to reconstruct Price’s schedule of trips. We matched the dates of Price’s trips to contracts for charter air service on USAspending.gov, the government database of all federal outlays. And we established the precise times of Price’s departures and arrivals through an exhaustive search of data we gathered from airports across the country. By Thursday, the matches were solid enough to go with a follow-up story: “Price traveled by private plane at least 24 times.”

As we looked at our growing database, we noticed some peculiarities about his trips. For instance, there was a Friday afternoon charter flight that took him to an island off the Georgia coast, even though he didn’t have any formal events scheduled for nearly two days. We checked property records, HHS financial disclosures and fundraising records from Price’s political career and realized the former Georgia congressman had long-standing ties to St. Simons Island: He and his wife regularly visited during the summers, both for fundraisers and to participate in the local medical association, and they owned undeveloped land worth more than $1 million there.

A day trip by Price to Nashville didn’t seem to make much sense as a priority for the HHS secretary, either. We could find only about 90 minutes’ worth of events on Price’s calendar, and many Nashville-area health care leaders told us they hadn’t even known he was coming. But once again, Price had a personal tie: His son lived in the city and the secretary himself owned property there. And the reporters were able to track down, and get HHS to confirm, that Price had lunch with his son that day.

That led to our next story: “Price’s private-jet travels included visits with colleagues, lunch with son.”

Though all our stories to this point were about taxpayer-funded charter flights, our sources insisted that his use of military aircraft bore scrutiny, too. Arguably, his use of military planes on trips to Europe and Asia—at a cost of more than $500,000, based on our review of invoices obtained through sources—was excessive for an HHS secretary, even if it was vetted by normal channels. Plus, the presence of Price’s wife, Betty, on the trips raised more questions. We followed up with HHS, the White House and scrutinized the travel practices of previous HHS secretaries before publishing our report on Thursday, September 28, “Price took military jets to Europe, Asia for over $500K.”

By then, the stories had become a national issue, with President Donald Trump among many other officials questioning Price’s decisions on travel. Our colleagues on POLITICO’s White House and Congress teams helped us keep track of the growing outrage. By Friday, September 29, Trump sent a strong signal that Price wouldn’t be around too much longer, telling reporters before he departed for his New Jersey golf club that they’d hear more about the fate of his HHS secretary that evening.

When word came of Price’s resignation later that afternoon, at 4:36 p.m., we weren’t sure what to do at first—we were in the middle of reporting a follow-up article. By that point, we had put in almost a thousand hours of work on the Price investigation. But we thought back to when we spotted that plane with the distinctive golden belly on the Dulles tarmac, just two weeks earlier, the moment that our slow-burn investigation became a real and consequential story.