Mark Zuckerberg today was confronted with what government attorneys characterized as a “smoking gun” in their antitrust case against : An email exchange in which he discussed the company’s rationale for purchasing other companies as a way to “neutralize a potential competitor.”

At the time, Meta had its eyes on fast-growing Instagram, and Zuckerberg and the company’s then CFO, David Ebersman, were having an exchange over the potential benefits of an acquisition.

“One way of looking at this is that what we’re really buying is time,” Zuckerberg wrote in the email, while also prioritizing another reason for the purchase, to integrate their products with Facebook‘s in order to improve their service. In the email, he also wrote that “even if some new competitor springs up…those new products won’t get much traction.”

On the stand, Zuckerberg went back and forth a few times with the FTC’s lead attorney, Daniel Matheson, over the emails.

Zuckerberg said that what he and Ebersman were doing a “build first and buy analysis,” weighing “whether we should buy companies in order to accelerate the development of our work.” He said that the idea was to leave those companies running and to improve them, as they became part of Meta’s offerings.

At the time, Facebook was building out an Instagram rival, Facebook Camera, but that was phased out after the company bought Facebook in April, 2012.

“I am sure we could have built our own app,” Zuckerberg said. “Whether it would have succeeded or not is a matter of speculation.”

The Meta CEO acknowledged that he was concerned about the scale that Instagram and other mobile apps were achieving, but that there were other factors.

Most of Zuckerberg’s morning testimony was taken up by Matheson showing him his internal emails and other messages, and asking the Meta CEO about them. Zuckerberg at times meandered a bit in explaining them, trying to point out that they lacked context to what is happening today or to how things eventually transpired with the company’s acquisitions. Rather than shut down Instagram, he pointed out, Meta improved it and greatly expanded its user base. But Matheson pointed out that there was a lack of documentary evidence that that was the rationale for the purchase.

The FTC sued Meta in 2020, claiming that its purchase of Instagram and of WhatsApp in 2014 were made out of a desire to eliminate competition and maintain its dominance in friends-and-family social networking. Meta has argued that the government has greatly narrowed the competitive landscape and ignored the rivalry from other social media giants like TikTok.

The flurry of emails throughout the morning included one that Sheryl Sandberg sent to Zuckerberg in November, 2012.

“I want to learn Settlers of Catan too so we can play,” she wrote.

“I can definitely tach you Settlers of Catan. It’s very easy to learn,” Zuckerberg wrote.

In the media room at the courthouse, the email generated laughter, even if Matheson didn’t refer to that part of the exchange in his questioning of Zuckerberg. Instead, Matheson referred to another part, where Zuckerberg expressed concerns that Facebook Messenger was not “beating” WhatsApp. He wrote, “Instagram was growing so much faster than us that we had to buy them for $1 billion,” while lamenting the slower progress of his company’s offerings Groups and Place.

“I don’t think it is accurate that the only reason we were interested in this was the scale or growth rate,” Zuckerberg told the court.