The company, which owns nationwide airwave licenses suitable for super fast 5G wireless service, the presumed successor to current 4G LTE mobile networks, said an unnamed bidder had offered $3.1 billion, beating the most recent previous proposal by almost $1 billion.

The newest bid of $184 topped the mystery bidder’s earlier bid of almost $136 per share from May 3, which beat that bidder’s initial bid of $104.64 on April 25, which topped AT&T’s publicly disclosed bid of $95.63 on April 10.

Investors and analysts expect more action, as can be vividly seen in Straight Path’s stock price, which shot far past the highest offer on the table to as much as $220.86 in midday trading on Monday, marking a more than 500% gain in the past month. In the year before AT&T’s April 10 bid arrived, Straight Path’s stock had bounced between $15 and $50, as investors debated whether its licenses in the 28 GHz and 39 GHz bands would ever become valuable and the company battled a probe by the Federal Communications Commission.

Get Data Sheet, Fortune’s technology newsletter.

Now the question seems settled: Straight Path’s licenses are worth plenty. AT&T has three days to top the bid from the mystery bidder, reportedly Verizon, and some analysts expect further increases after that.

“While VZ likely will not comment on the STRP deal, it is clear to us that this spectrum asset represents a critical pillar in its 5G strategy,” analyst Jennifer Fritzsche from Wells Fargo noted on Monday, using the stock ticker symbols for Verizon and Straight Path. “Given the limited availability of this asset, we believe VZ (assuming it is the second bidder) will continue to aggressively pursue this purchase even if T comes back with a new offer within the next 3 days.”

Verizon (VZ) and AT&T (T) declined to comment on Monday. Straight Path (STRP), in a statement, said the unnamed bidder had agreed to cover a $38 million termination fee owed to AT&T if the company accepts the higher bid.

Straight Path is one of the few owners of airwaves that are suitable for upcoming 5G wireless networks, which will be fast enough to allow downloads at 10 to 40 times the speed of current 4G LTE networks. The faster network would be suitable for mobile virtual reality applications or transmitting masses of data from corporate connected machinery, for example. Verizon and AT&T are also conducting trials to use the 5G spectrum for serving homes and businesses with ultra-fast Internet and video connections.

Known as the millimeter bands, the higher frequencies covered by Straight Path’s licenses are capable of carrying far more data than lower bands like the 700 MHz currently used in 4G wireless networks. But they have trouble traveling as far and penetrating obstacles.

The bidding war between AT&T and Verizon “means that the carriers are serious about their efforts in moving ahead with millimeter wave spectrum,” writes analyst Craig Moffett of MoffettNathanson. “In Verizon’s case – assuming that it really is Verizon bidding here – it may not be an overstatement to suggest that millimeter wave may soon be the foundation of their entire strategy.”

The rising value of the licenses could also aid T-Mobile (TMUS), which inherited substantial holdings in the 28 GHz band when it bought MetroPCS four years ago, Moffett noted.

The licenses owned by Straight Path have traveled a tumultuous journey. Telecommunications carrier IDT (IDT) bought the rights out of the bankruptcy of Winstar, a failed wireless service, back in 2001. Unable to extract much value from the licenses over the next decade, IDT spun off the rights with Straight Path in 2013.

Straight Path then faced a FCC investigation over whether it was putting the licenses to proper use. In a deal reached with the FCC in January, some of the licenses were revoked, and Straight Path agreed to pay the agency 20% of any sale.