Apple’s AI and search teams are continuing to churn, as Siri cofounder Tom Gruber and search chief Vipul Ved Prakash have now left the company. As originally noticed by the Information, their departures coincide with John Giannandrea taking over as Apple’s chief of machine learning and artificial intelligence.

Gruber was the last of three Siri cofounders to remain at Apple, sticking around well after the departures of former partners Dag Kittlaus and Adam Cheyer, who left to start Siri competitor Viv. Until his departure, Gruber served as the head of Siri’s Advanced Development Group, which has clearly struggled over the years to achieve either parity or a leadership position relative to other AI assistants. He is reportedly retiring from Apple to “pursue personal interests in photography and ocean conservation.”

Prakash was previously CEO of search engine and analytics firm Topsy, which was purchased by Apple in 2013. He was made head of search for Apple, an interesting position given the company’s complex relationship with Google and Microsoft, which it has alternated between for operating system-level search functionality while working on internal solutions. Today, Apple uses its own search engines for app and media discovery in its online stores, but continues to hand off other searches — including Siri’s and Safari’s.

Siri has been one of Apple’s most persistent and public development challenges over the past decade, having already moved from key executives Scott Forstall to Eddy Cue to Craig Federighi before recently becoming Giannandrea’s responsibility. Reports have blamed everything from poor strategy to a problematic software infrastructure for the AI assistant’s failings, which contributed to mediocre reviews of the HomePod speaker early this year.

Apple has reportedly been on a hiring spree to improve Siri, posting jobs for numerous engineers and related positions. In April, the company hired Giannandrea from Google, where he was the head of search and AI, and then consolidated AI and machine learning divisions under him earlier this month.