They're from Arizona, Massachusetts, and California, according to KIRO:

Robert Lehner, Chief, Elk Grove Police Department
Lehner has 30 years of law enforcement experience and previously served as the police chief in Eugene, Ore., according to his biography. Prior to that, he served as an assistant police chief for the Tucson Police Department. Read more about Chief Robert Lehner here.

Patrick Melvin, Chief, Salt River Police Department
Melvin served for 21 years on the Phoenix Police Department before retiring at the rank of commander, according to his biography. He went on to serve as police chief in Maricopa, Ariz. Read more about Chief Patrick Melvin here.

Frank Milstead, Chief, Mesa Police Department
Milstead has led the Mesa Police Department since 2010, following a 25-year career with the Phoenix Police Department, according to his biography. Read more about Chief Frank Milstead here.

Kathleen O'Toole, former Police Commissioner, Boston Police Department
O'Toole began her career in law enforcement in Boston in 1979, according to her bio. She rose through the ranks to become Boston Police Commissioner in 2004. She currently heads a private consulting firm in Boston. Read more about Kathleen O'Toole here.

"The candidates that we have now are all reformers," police chief search committee co-chair Pramila Jayapal, who confirmed the names, tells me by phone. "They have all, in different ways, been very involved in building or transforming police departments...none of them is a sit-on-your-hands, take-whatever-comes kind of person."

I asked her whether the Seattle Police Officers' Guild (SPOG), which has grown in influence and set back the reform process under Mayor Ed Murray, has had an impact on the search so far—SPOG President Ron Smith sits on the 12-member search committee. "So far in the committee process, they [SPOG] haven’t been different than anybody else," she says—in fact, she couldn't recall a single time Smith had disagreed on something with her.

"Of course, once we get somebody in and they make the changes they need to make, we’ll see what happens then," Jayapal says. "But from my perspective, the discussions have been pretty uniform." She says they'll continue to vet the candidates in the coming weeks, then recommend the best three to Murray, who expects to choose one by mid-May.