AT&T will help build a coast-to-coast public safety network after all 50 states opted into the FirstNet public-private authority by last week's deadline.

New Hampshire previously signaled plans to opt out of FirstNet in favor of a plan from Rivada Networks, but changed course on Thursday after it became apparent all other states and the District of Columbia would pick AT&T's plan.

Puerto Rico and the U.S. Virgin Islands also opted into FirstNet, while three other territories — Guam, American Samoa and the Northern Mariana Islands — have until March to make their decisions.

“Our FirstNet offering will forever change the way first responders communicate,” Chris Sambar, who heads FirstNet operations for AT&T, said in a statement. "Securing 53 opt-ins is significant for the public safety personnel that this network will serve."

The authority said first responders in those states and territories will gain immediate access to AT&T's network, including priority and preemption features during high-traffic or emergency situations.

Officials said the new year would likely see "aggressive" network construction, including site installations to help cover unserved or underserved areas, to expand FirstNet and deploy Band 14 capacity.

FirstNet will provide AT&T with access to 20 MHz of nationwide 700 MHz spectrum, along with about $7 billion in government funds to help with build costs. Wells Fargo Senior Analyst Jennifer Fritzsche wrote in a note to investors that FirstNet construction would affect as many as 15,000 cell sites in the first year and about 45,000 over a five-year period.

“We see this as a significant boon for the tower industry – beginning in 2018 and extending years after,” Fritzsche wrote.

FirstNet added that its public safety core infrastructure with end-to-end encryption is expected to begin operating in March, while AT&T plans to begin offering a dedicated security center and a host of new public safety tools, such as "mission-critical push-to-talk and device offerings, like BYOD."

"2017 was about planning, preparing and moving quickly to bring public safety a meaningful option – one they can rely on without delay," Sambar said. "We’ll build on that groundwork in 2018 with transformative capabilities that will make FirstNet the most valuable communications system for first responders."

Critics, who raised concerns about FirstNet's transparency and flexibility over the life of its 25-year contract, had encouraged states to opt out of FirstNet by Thursday's deadline in order to consider alternative operations.

New Hampshire Gov. Chris Sununu maintained that Rivada's plan was "the better option" for the state, but that the regulatory and financial risks of being the sole state to opt out of FirstNet "creates too high a barrier."

“I applaud these governors for their decision and congratulate public safety for its advocacy and partnership throughout the process," FirstNet CEO Mike Poth said in a statement.