AT&T and Verizon are again being investigated for collusion. Here’s what happened the first time.

The Justice Department is investigating whether AT&T and Verizon may have colluded to thwart a technology that could allow wireless customers to switch network providers more easily. But antitrust officials conducting a similar probe in 2016 found little evidence for the claim and ultimately dropped the inquiry, according to a letter sent by the Justice Department. In closing the Obama-era investigation, the government said that while mobile carriers had pushed for new industry policies that raised competitive concerns, their initiative proposed little more than a “technical capability” for cellphones without clearly demonstrating how or if companies such as AT&T and Verizon might use it to gain an unfair advantage.

The investigation could be reopened if the proposed policies raised further concerns after implementation, the Justice Department indicated in the letter. While some of the policies have been approved for inclusion in the final standard governing cellphones, the overall policy remains in the drafting phase and has not yet been implemented.