WASHINGTON — Since Democratic National Committee officials first discovered their data networks had been compromised this spring, a growing chorus of experts and officials have seen evidence that the Russian government was responsible.

In the months since, the infiltration and its consequences have taken surprising and often bizarre turns, culminating in a political scandal this week as the Democratic National Convention opened in Philadelphia. But one constant has remained: a growing body of forensic evidence implicating the Russian government.

The first hints came in May, after committee officials noticed unusual activity in their network. They hired the cybersecurity company CrowdStrike to investigate, and its experts quickly found the source of the activity: a group of hackers had, in late April, gained access to the systems of the committee’s opposition-research team, from which the group had stolen two files containing information on Donald J. Trump, who would eventually become the Republican nominee for president.

The investigators determined that the hackers were part of APT 28, a group well-known among cybersecurity experts. The name is short for advanced persistent threat, which usually refers to government hackers. Security firms and law enforcement officials have also used the name Fancy Bear, a reference to a widespread belief that the group is run by Russia’s military intelligence agency, the G.R.U.