Tesla continues to develop its heavy-duty semi electric truck, but it’s on the backburner behind the Model 3 sedan, according to tweets from CEO Elon Musk.

During a weekend Twitter exchange with Electrek’s Fred Lambert, Musk confirmed that Tesla’s Jerome Guillen continues to apply his electric truck know-how to that project. However, completing the soon-to-be-launched Model 3 is still top priority for the development team, Musk said.

There’s been a lot of interest in a Tesla electric commercial truck ever since Musk posted his second version of the “master plan” last summer. He said that early versions of electric trucks and buses could be released this year.

Guillen came to Tesla in 2012 after working for Daimler and playing a leading role in its venture with Freightliner through the U.S. Department of Energy’s SuperTruck initiative. That heavy duty test truck rolled out in 2010 and earned a 12.2 miles per gallon fuel economy rating – double the DOE’s initial goal for the Super Truck program.

In September, Electrek also reported that Tesla hired several key Daimler engineers who also worked on the same program, including Evan Chenoweth. He had been the lead designer of Freightliner’s SuperTruck hybrid engine, and how serves as a Tesla senior mechanical design engineer.

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Clean semi trucks, and urban medium-duty trucks, are gaining a lot more support and credibility as the U.S. government carries out fuel economy and emissions rules. Mercedes-Benz last year introduced its Urban eTruck, which the company says can go up to 124 miles on a single charge, making it ideal for urban delivery and short hauls while meeting clean air rules.

Nikola Motor Co. continues to gain attention for its Nikola One zero emission heavy-duty truck. It’s powered by a lithium-ion battery pack and hydrogen fuel cells. The company estimates truckers can go up to 1,200 miles on a fully charged and fueled Nikola One.

Toyota launched a test project last year combining its love for hydrogen fuel cell technologies with its interest in expanding to new markets. In November it was revealed that the Japanese automaker has been working on a hydrogen fuel-cell semi truck project.