Fire erupts after an incident at a Houston chemical plant, and I know who to call with my questions.

I pick up the phone to reach Dr. M. Sam Mannan.

It's 3 a.m. in another country thousands of miles away. It could be South Korea, or Hong Kong, the Netherlands or Dubai.

Despite the distance and the late hour, even though I'm a reporter, even though many people view journalists as contemptible and enemies of the people, Mannan picks up the phone.

"Hello, Matt. How are you?" he answers in a sleepy, but cheerful voice.

In short order, I find out how late it is for him. I apologize profusely. Mannan traveled frequently, and it was always difficult to know where he would be at any given moment. But he waves away the inconvenience, ready to answer my questions again.

"Oh, don't worry about it, I'm always happy to talk to you, my friend," he'd say.

There was a reason Mannan was first on my list to call when a chemical plant was on fire or exploding.

***

THE RELATIONSHIP between a reporter and a source can be a little strange. From the outside, it's transactional. I need information from my source. My source gives me information. I write the story. It gets published. Relationship over, until we do it all over again later for another story.

Sometimes, it goes beyond that.

And that's what it was like with Mannan and me.

Mannan recently died at the age of 63, and I can't stop thinking about him. He was a world-renowned expert on chemical process safety. He spoke at conferences, was an author of more than 300 academic papers and served on the Columbia shuttle investigation in 2003. He was a professor at Texas A&M and led the Mary Kay O'Connor Process Safety Center, named after one of the victims of the deadly 1989 Phillips 66 explosion.

He was a husband to his wife, Afroza, and father to two daughters, Joya and Rumki.

For me, Mannan was a source. But he was more than that, too.

***

OUR RELATIONSHIP started on YouTube. It was early 2015. I was learning everything I could about the chemical industry for a project I was working on. After more Google searches than I'd like to admit, I found Mannan.

He was speaking to a U.S. Senate committee in a C-SPAN video, his glasses and bald head shining in the bright lights of the hearing room. In his testimony, he was laying out the ideas that I had in my head but couldn't articulate clearly yet.

When he didn't answer his phone or emails, I tracked him down at a chemical safety conference in Austin. He was giving a talk there, and I figured I could chase him down afterward.

My colleague Mark and I surrounded him as he left the restroom, and we proposed something preposterous. We wanted Mannan to help us show people which chemical plant was at the biggest risk for an incident.

To our surprise, he immediately said yes, but with a condition.

Mannan told us we'd never be able to do what we wanted, but he had a better idea if we were open to talking it through. We set an appointment to talk in College Station in a week.

When we arrived at A&M, Mannan had pulled together an entire team of student researchers and a buffet of barbecue.

We came with an idea of a project and left with a plan for a detailed analysis that no one had ever done before. This was not normal. Reporters are lucky if we get calls back. I can't tell you how many emails or phone calls go off into the ether only to be ignored.

We had more than a source. We had a guide.

***

"CONSIDER THE knife," Mannan said to me on more than one occasion.

We were talking about a chemical safety concept called inherent safety. It means instead of trying to manage potentially dangerous chemicals, companies should limit the chemicals they have on site, because that's inherently safer.

But Mannan liked to tell the story of the knife to illustrate the limits to that concept.

"A knife is useful for cutting vegetables or meat, but that same knife can be a deadly weapon or could accidentally hurt or kill someone," he said.

Getting rid of knives in a kitchen obviously wouldn't work. You need to cut things in a kitchen. Instead, you have to learn how to use a knife safely.

"We have to remember nothing can be inherently safe on an absolutely basis," Mannan said. He took ideas that were complicated and technical and made them easy to understand. He was an academic who didn't live in an ivory tower. Mannan wanted his work and research to be used and understood, not just by the industry he once worked in, but by the general public.

The Process Safety Center at A&M had large number of graduate students who weren't from the U.S. Mannan and his fellow faculty took care of those students while they were so far from home. He created a family atmosphere and encouraged them to do their best work.

Trish Kerin knew Mannan professionally. She's a chemical expert with the Institution of Chemical Engineers. Kerin saw firsthand how he inspired his students to rise to the occasion and produce their best work.

Reporters don't work for their sources. But in a real way, Mannan inspired me to do my best work, to show I learned the material he presented to me.

"You wanted to make him proud and he always was," Kerin said.

***

HE WASN'T born and raised in the U.S., but I can only think of handful of people who embodied what it was to be American as well as Mannan.

When "Chemical Breakdown" was published, the industry was not happy with the Chronicle, but it was especially not happy with Mannan. He had worked for the industry. He spoke at their conferences.

We knew there would be pushback. We tried to warn Mannan about it. He waved the concerns away. He told us he could "handle any heat I get for this."

"Your project was uncomfortable for him at times," said Kerin.

In a meeting with donors to Texas A&M, including the members of the Texas Chemistry Council, Mannan was criticized as Kerin watched. They wanted to know why he worked with the media on a project that was critical of the industry.

But Mannan pushed back.

He told them it was his duty as an academic to do this work, that the Chronicle would be abdicating its responsibilities to journalism if we didn't do this work. He said he and the Chronicle had to speak truth to power without fear or favor.

"You could always trust him to do the right thing and not the easy thing," Kerin told me. "He challenged people's thinking. Challenged them to do better. To be better."

***

WE HAD multiple conversations after "Chemical Breakdown" was published, mostly about stories I was working on. But we always made time to talk about our families and our friends and how life was going.

The last time I talked to Mannan was after Arkema was criminally indicted for not preventing the chemical fires at its Crosby site in the aftermath of Hurricane Harvey.

I wanted to hear his thoughts on such a rare thing, a criminal prosecution of a chemical company. "I regret to tell you, Matt, that I cannot speak on this case as I am an expert witness for the county," Mannan said in his particular formal tone of voice.

He chuckled after he said it. I told him that was a shame, but I was sure we would talk again soon regardless.

We frequently talked about getting together in College Station again, getting a steak and celebrating our collaborations over a beer or two. We tried scheduling it a few times, but life got in the way, and we kept having to move the date.

I'll never forget the man who'd answer my calls at 3 a.m. in a city far from home. The man who could make chemical safety about a knife in a kitchen. The next time something breaks down in the chemical industry, I'll reach for the phone to call a source, but I'll stop myself in the moment, because it won't be my friend Sam on the other end of the line.

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