By Mike Colias

A Nikola Corp. executive defended the electric-truck firm's technology and business plan, telling an investor conference that its partners are staying the course following the departure of its high-profile founder and executive chairman.

"Nothing has changed," Kim Brady, Nikola's chief financial officer, said during a virtual conference Tuesday. "We recommend that investors really focus on the future and what we have delivered and what we're going to deliver."

Nikola, a Phoenix-based startup with backing from General Motors Co., Robert Bosch GmbH and others, has come under scrutiny recently following allegations it misled investors about the progress of its technology.

The allegations were outlined in a short seller's report released earlier this month. Nikola has called the report by Hindenburg Research false.

Nikola founder Trevor Milton stepped down from his role as executive chairman Sunday, saying he wanted the focus of the company to be on the business, not him. He was replaced by Stephen Girsky, a former GM executive and Nikola director, who is well-known in the auto industry.

The allegations about the startup have raised questions among investors and analysts about whether Nikola's partner companies did their due diligence.

GM Chief Executive Mary Barra said the auto maker conducted "appropriate diligence" when she was asked about the topic on a recent analyst call.

Mr. Brady on Tuesday said Nikola has been well vetted, and that Bosch, for example, sent an "army" of engineers to assess the company and its technology before making its investment.

Bosch said it performs due diligence as part of its investment strategy but that the details of the process are confidential and proprietary. Earlier this week, the Gerrman engineering group said it intends to continue working with Nikola.

Among Hindenburg Research's allegations was that Mr. Milton led investors to believe that Nikola largely develops its own technology when in fact it purchases it from outside suppliers.

"We have never represented that we make every component," Mr. Brady said at the conference Tuesday, hosted by Evercore ISI.

"Think about Apple iPhones," he said. "They don't manufacture a single part. But they control design and functionality and user interface."

At the heart of the short seller's report is whether past statements from Mr. Milton gave investors a false impression of Nikola's progress. The scrutiny also has raised fundamental questions about what technology the company has developed on its own that will end up in its trucks.

Nikola's business model is built around the concept of semi-trucks powered by batteries and hydrogen-fuel cells. The company says it plans to partner with energy companies to build hydrogen fueling stations, while contracting with manufacturers to help engineer and assemble some of its semi-trucks. It plans to lease the trucks to freight operators, promising to lower their operating costs.

Nikola is developing a few prototype trucks, with its first model expected to go into production by the fourth quarter of next year. It also struck a deal with GM this month to have the auto giant engineer and build a zero-emissions pickup truck, a market that many startups and traditional car companies are targeting, including GM with its own model.

It is common for car makers to outsource the manufacture of parts and even larger systems of a vehicle. Much of the key technology in GM's Chevrolet Bolt electric car, for example, is supplied by Korean electronics company LG Chem.

The Hindenburg Reseach report took Mr. Milton to task for what it claims are false statements about Nikola having itself developed certain key components, including batteries.

Mr. Brady said it is common for auto makers to get their battery cells from suppliers such as Panasonic Corp. or Samsung Electronics Co. At Nikola, he said, "we try to control" the design of the battery pack in which the cells are housed.

"We do our own batteries at Nikola and have since day 1," Mr. Milton said in a tweet on June 4, the day the company went public. "We do, however, test with everyone, and if a supplier builds a better battery, we would use them."

The Wall Street Journal and the Financial Times have reported that the company plans to use California-based Romeo Systems Inc. to provide batteries for its first semi-truck.

Nikola went public in June through a reverse merger, part of a wave of investor enthusiasm in recent months for electric-vehicle startups. The company's valuation initially soared -- briefly edging past that of Ford Motor Co. -- -- before falling sharply in the wake of the short-seller report.

Nikola shares were up about 3% on Tuesday after dropping Monday in the wake of Mr. Milton's departure.

Mr. Brady also defended against Hindenburg's allegation that Nikola misled investors by claiming that trucks displayed on stage at past events, and in a 2018 promotional video, were operational even though they were unable to propel themselves.

Nikola has said the truck shown in the video had functional batteries and other parts, but said it wasn't propelled on its own. The video never stated the prototype was driving under its own power, the company said, adding that its newer trucks are able to be driven.

"Anyone who's involved in the automotive world understands when a truck is first introduced, it's a show truck," Mr. Brady said.

The Securities and Exchange Commission and the Justice Department are investigating whether Nikola misled investors by making exaggerated claims, people familiar with the matter have said.

"That's what we've heard," Mr. Brady said about the reported probes. "When we have more to disclose, we'll disclose."

Write to Mike Colias at Mike.Colias@wsj.com