Google didn't immediately respond to questions about how or whether it plans to revise its transparency tool to provide that kind of information.
"In the online space, we're pretty much at the mercy of the platforms as to what they choose to make transparent," said Damon McCoy, a computer science researcher at New York University who studies election integrity and who has pushed tech companies to disclose more.
The focus on Google's ad transparency tool follows the company's announcement last month that it is tightening its policies on political ads. The company said it will no longer allow political ads to be targeted to specific groups of voters by ideological affiliation or by information found in public voter files.
Critics of highly specific political ad targeting have argued that, without sufficient transparency, the tactic could allow malicious actors to secretly and deliberately mislead voters on key issues. Ellen Weintraub, the chair of the Federal Election Commission, has called on tech companies to voluntarily end the practice.
Twitter has also announced plans to ban politicians and SuperPACs from advertising on its platform. Facebook has drawn scrutiny for allowing politicians to run false ads.