Shares of Straight Path shot up again after as the bidding war for the company between AT&T and an unidentified telecom—likely Verizon—continued to escalate.

AT&T earlier this month announced plans to acquire Straight Path Communications in a deal aimed at bulking up its portfolio of millimeter-wave spectrum. But reports surfaced last week indicating a major telecom player—said to be Verizon—had outbid AT&T, agreeing to pony up $1.8 billion.

AT&T said last week that it would reconsider its offer in the coming days and may decide to up its bid. Straight Path hasn’t identified the second bidder, but the company said last Wednesday that it had received a revised offer “from a multinational telecommunications company” for $135.96 per share, or $2.3 billion.

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That bidder submitted a revised offer, Straight Path said this morning, valuing the company at $3.1 billion. AT&T has three days to match or exceed the current bid.

Straight Path stock soared nearly 30% on the news by mid-day Monday.

Straight Path earlier this year reached a $100 million settlement with the FCC, ending an investigation into the company’s failure to deploy wireless services as required under FCC spectrum licensing rules.

Straight Path agreed to pay two civil penalties and surrender about 20% of its 5G licenses to the FCC. For the $100 million civil penalty, Straight Path will pay $15 million upfront with $85 million suspended unless Straight Path sells all its remaining licenses or surrenders them to the FCC within 12 months. Also, 20% of any sale proceeds must be paid to the Treasury as an additional civil penalty.

Post-settlement, Straight Path holds an average of 620 MHz in the top 30 U.S. markets and covers the entire nation with 39 GHz spectrum. It has retained all its 28 GHz spectrum licenses. Straight Path had long been expected to sell its licenses to a wireless or cable network operator.

The bidding war underscores the ever-increasing value of millimeter-wave spectrum as mobile network operators plot their strategies in advance of the launch of 5G offerings. Although higher-band airwaves have historically proved difficult to use to provide wireless services, they do promise to allow carriers to provide much higher speeds and far greater capacity, albeit in smaller coverage areas.

Indeed, high-band airwaves appear to be much more highly prized lately than low-band spectrum, particularly among the nation’s two largest carriers. AT&T committed to spend only $910 million in the FCC’s incentive auction of 600 MHz spectrum, for instance, and Verizon opted to spend nothing despite qualifying for the event.