Top aides to Prime Minister Justin Trudeau on Tuesday said that meddling by foreign powers into internal Canadian affairs was “an evolving landscape” and pushed back at suggestions that the government had been slow to respond.

The aides spoke at an ongoing public inquiry in Ottawa focused on interference by foreign nations a day after Canada expelled six Indian diplomats it accused of orchestrating killings inside Canada.

“There has been step after step after step that has been taken, and there are more steps that need to be taken,” said Katie Telford, who is Mr. Trudeau’s chief of staff and considered one of the most powerful officials in the Canadian government.

India has forcefully denied that its officials were involved in the killings.

While Ms. Telford acknowledged there were “vulnerabilities” that facilitated interference, she rejected suggestions that political parties’ internal elections were one of them. In a preliminary conclusion, the inquiry found that there were “gateways” to interference, and a special parliamentary committee’s report said that internal party elections had created a “critical gap” in Canada’s democracy.

Mr. Trudeau is scheduled to testify Wednesday before the inquiry, which the prime minister had long opposed. But mounting evidence of foreign interference in the past two general elections and an extraordinary series of leaks by Canadian intelligence to the news media forced Mr. Trudeau to establish the inquiry.