BARBARA HOWARD: Campaigning for mayor of Boston costs money. WGBH Radio's Isaiah Thompson set out to trace the source of campaign funds. He's with me in the studio. Hi, Isaiah.

ISAIAH THOMPSON: Hi, Barbara.

BARBARA HOWARD: So you look not just at individuals, but at companies financing the Boston mayoral runs between the sitting mayor, Marty Walsh, and his challenger, City Councilor Tito Jackson. Tell me what you found.

ISAIAH THOMPSON: Right. So, it's important for us to point out that corporations, companies, cannot make donations to political candidates — only individuals can, but individuals who donate more than $200 do have to disclose who their employer is. And what I found is, if you step back, you take a look at the employers that turn up the most often in this data, you see distinct patterns. You see companies whose employees have made lots of donations year after year totaling large sums of money.

BARBARA HOWARD: Are you saying it's in a concerted way that somebody is actually orchestrating this? I mean, how do you know?

ISAIAH THOMPSON: No, there's no way to know that. I mean, in fact, companies are not allowed to sort of reimburse, for example, that would be illegal. But what we do see is a lot of individuals, often owners, you know, high up people — not always — from these companies really making, you know, contributions year after year for large amounts of money and sometimes totaling tens of thousands of dollars.

BARBARA HOWARD: So which companies come to mind?

ISAIAH THOMPSON: So, a few examples. One would be CBRE New England. They're one of the biggest real estate firms around and their employees have made more donations to Walsh over time than those of any other private company. In this case, as in other cases I looked at, this is a company that also has direct business with the city. For example, CBRE has an arrangement with the Boston Redevelopment Authority to market its properties and to do some leasing work for them. Now that started under former Mayor Menino, I should point out, but that contract, that arrangement, has continued under Mayor Walsh.

BARBARA HOWARD: And are there any other companies?

ISAIAH THOMPSON: Yes. Just to give one more, there's a company called Feeney Brothers. Their employees made a total of about $20,000 in donations to Walsh over the last four years. That company has been paid millions of dollars in a snow removal contract for the city.

BARBARA HOWARD: OK, so, but this is not just about Mayor Walsh, right?

ISAIAH THOMPSON: No, and in fact, if you look at the companies whose employees have donated the most to Walsh’s challenger, Boston City Councilor Tito Jackson, you will also find a few real estate companies who have had projects that needed approval from the Redevelopment Authority. Now, of course, one difference there is that Jackson doesn't appoint the Redevelopment Authority, Walsh does. Walsh and Jackson have both said that these donations don't influence their decisions. And it's important to remember, in the big scheme of things, these are still pretty small portions of the money that either of them has raised. But there is still the question of appearances.

BARBARA HOWARD: Have you had a chance to talk to either Walsh or Jackson about your findings?

ISAIAH THOMPSON: Yeah, I reached out to both campaigns. The Walsh campaign sent a statement saying, in essence, that they review all contributions and that they are in full compliance with the law. Tito Jackson sent a statement critical of Mayor Walsh. He pointed out that Walsh promised to make city lobbying activity public when he came into office. That still hasn't happened, although Walsh did get behind a plan to do that earlier this year.

BARBARA HOWARD: OK, thanks for joining us, Isaiah.

ISAIAH THOMPSON: Thanks for having me.