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Senator Bernie Sanders, looking to shore up his credentials on foreign policy, is seeking advice from a patchwork of experts, including Lawrence Wilkerson, who was chief of staff to Secretary of State Colin L. Powell and who has speculated that Israel might have carried out chemical attacks in Syria.

Mr. Wilkerson, a former Army colonel who has become an ardent critic of the American invasion of Iraq in 2003, speculated about Israel’s possible role in a May 2013 interview with Cenk Uygur of Current TV.

“This could’ve been an Israeli false-flag operation, it could’ve been an opposition in Syria,” he added, saying that the attacks also could have been carried out by Bashar al-Assad, the Syrian president. (The United States believes the Syrian government was in fact behind the chemical attacks.)

“But we certainly don’t know with the evidence we’ve been given,” he added. “And what I’m hearing from the intelligence community is that that evidence is really flaky.”

In an interview on Friday, Mr. Wilkerson was not remorseful about suggesting that Israel might have played a role in the attacks. But he said he was merely listing those who could have played a part, and wasn’t trying to fan flames against Israel.

“I was just suggesting all the different people that could have been involved at a time when speculation was rampant,” said Mr. Wilkerson, adding that there was still some debate within the intelligence community about exactly what took place.

Demonstrating an ability to show foreign policy depth has been a challenge for Mr. Sanders, who has promised to roll out a list of advisers on the national security front.

An aide to Mr. Sanders did not respond to a request for comment on Friday.