Can business-to-business e-commerce become Amazon.com's (AMZN) - Get Report fourth growth pillar? 

As Amazon searches for a fourth pillar aside from Prime, cloud computing division Amazon Web Services and its third-party Marketplace, court testimony by its senior executive in the antitrust litigation offers an insight to the e-commerce giant's interest in the industrial distribution vertical, RBC Capital Markets analyst Deane Dray wrote in a Thursday note.

In particular, Jeff Bezos' empire appears to be going after business products distributor W.W. Grainger(GWW) - Get Report .

"We believe the long shadow of Amazon Business has been a growing threat to Grainger ever since Amazon entered industrial distribution in 2012," Dray wrote.

Launched in 2015, Amazon Business replaced AmazonSupply.com -- a business-to-business e-commerce marketplace. The division sells products in a wide range of categories including office products or electronics for businesses of all sizes.

Amazon Business vice president Prentis Wilson appeared in federal court earlier this year during antitrust proceedings for the proposed merger between Staples (SPLS) and Office Depot(ODP) - Get Report .

The two office suppliers agreed to a merger in February 2015, but the Federal Trade Commission quickly raised concerns about the tie-up on the basis that the transaction would drive up prices by suppressing competition. Staples subpoenaed Amazon during the litigation in an effort to illustrate that online retailers were posing a serious threat to the industry, and that a Staples-Office Depot marriage wouldn't be anticompetitive. The merger was called off in May.

The cross-examination of Amazon's Wilson underscored Amazon's long-term strategy for B2B in addition to illustrating the threat this mindset is increasingly posing to Grainger, Dray explained. The Lake Forest, Ill.-based industrial supply company may be the most vulnerable because Grainger and Amazon both primarily target small and medium-sized businesses.

Meanwhile, Amazon has been quietly but surely building out its distribution capabilities.

The Seattle-based e-commerce giant has identified Amazon Business as a top priority, and internal documents have specifically highlighted Grainger and Staples as competitors, Dray pointed out.

"Amazon has a sense of urgency to 'leverage the Amazon brand' and to 'land grab' in the competitive distribution environment," he said. "Seamless transition from Amazon.com to Amazon Business is an advantage. This was emphasized as an important point, as many customers, whether small businesses or individual employees at large businesses, are familiar with Amazon.com and thus feel very comfortable taking the leap to Amazon Business."

Still, Amazon may end up partnering with Grainger rather trying to directly compete against it, Tigress Financial Partners Ivan Feinseth said via phone Friday.

While Grainger is known for its high-quality customer service, Amazon excels in pricing power and efficiency, Feinseth argued, while acknowledging that Amazon will continue to put emphasis on supply chain management.

In fact, Amazon has been steadily building out its own transportation and logistics network in an effort to improve shipping for customers, particularly Prime members.It most recently showcased its first-ever air cargo plane during Seafair's Air Show in Seattle, and also sealed a partnership with Atlas Air Worldwide Holdings(AAWW) - Get Report  to lease planes earlier this year.

"Amazon is going after any and all markets they see [as] vulnerable," Benchmark analyst Dan Kurnos wrote in an email, adding that Amazon should be able to make inroads in industrial and office supplies by taking advantage of changes and chaos in the broader retail market.

Still, he cautioned that Amazon has its fingers in a lot of pies and it's hard to gauge how strongly it plans to push into this vertical.