Matthew Monahan, the CEO of big-data startup Inflection, had to wake his brother and co-founder, Brian, in the middle of the night, so he could witness the signing of a $100 million deal in which family history site Ancestry.com agreed to buy Archives.com.

The deal, which the companies have just announced, was signed at 4:58 am after a marathon 72-hour closing process. The acquisition gives Ancestry.com an important vertical in the growing family history category and a new team of engineers and digital marketers from the Archives.com team. Ancestry.com will also gain access to the proprietary technology platform that has supported Archives.com’s rapid growth. The deal is worth about $100 million in cash and liabilities.

Monahan, who was on nightwatchman duty, captured the moment with his video camera. He said the companies had been discussing a possible acquisition on a conceptual basis for a couple of months, but “mountains were moved” over the past few days.

Despite the huge sale, it’s a bittersweet moment for the Utah-based startup, because it will soon lose some of its core staff, who will be moving to Ancestry.com’s San Francisco offices. However, Monahan says that Ancestry.com has promised to continue to invest in Archives.com’s core mission, which is to make family history simple and accessible for a mass market.

“It’s one of those rare opportunities, where as an entrepreneur, you want to see products you build have a life, have a home, and be able to continue,” Monahan said.

Since Archives.com’s launch in January 2010, the site has grown to more than 380,000 paying subscribers, each of which pays about $39.95 a year. The service offers access to more than 2.1 billion historical records, including birth records, obituaries, immigration and passenger lists, historical newspapers, and US and UK Censuses.

Monahan also dropped a hint about a new product in the works from Inflection: Identity.com, an online identity monitoring and management tool that Inflection describes with the tagline “Your life. Your work. Your choice. Shape your online identity.” No word yet on when it’s going to launch.