NEW YORK, Oct 28 : The Federal Election Commission restored its electronic filing system Wednesday night after taking it offline for nearly 41 hours to contain a cyberattack, which temporarily concealed spending from big donors as political donations ramp up in the final days of the midterms.

Campaign watchdogs sounded alarms as the outage dragged into its third evening. Super PACs and other groups could not file mandatory 24- and 48-hour reports that show how much money they are spending to help or hurt candidates at the polls, shrouding the most recent political spending from public view.

"We didn't have access to the last-minute spending, which is really ramped up as the election draws nearer," said campaign finance watchdog Molly White, who tracks crypto and Big Tech political spending. "We have no visibility into that when the filing systems are down." "It's pretty devastating this close to the election," she added.

The outage came at a critical time as daily spending accelerates to its fastest pace in the two-year election cycle, campaign finance experts said. Super PACs, which can accept and give unlimited amounts of money, and dark money groups spent an average of over $50 million per day the week before the outage, OpenSecrets research director Brendan Glavin said. More than $250 million in spending was reported on October 20 alone, he added.

The FEC told users that most of the services affected by the outage have been restored. It posted a notice just after 10 p.m. ET on Monday saying it was investigating a problem that caused "major" outages across its systems that process electronic campaign filings.

Reuters could not establish the motive for the hacking effort. Hackers of all stripes regularly target government networks for espionage, financial gain or political motives.

The FEC does not play a role in administering US elections and the outage should have no effect on voters' ability to cast ballots.

“Disclosure delayed is disclosure denied,” Hilary Braseth, executive director of nonpartisan campaign finance watchdog OpenSecrets, said in a statement released before the system was restored. “When hundreds of millions of dollars in outside money flood political airwaves in the final days before an election, transparency is not an optional luxury, it is a fundamental public safeguard.”The FEC did not return requests for comment Wednesday and did not announce any details on the cause of the outage. The Cybersecurity and Infrastructure Security Agency and the FBI both referred questions to the FEC.

An FEC officer told campaign compliance firms late Monday that it took those services offline after a "bad actor" tried to breach the agency's systems, according to an email reviewed by Reuters.

Campaign finance managers had to reset their passwords before logging in to the system, according to an email reviewed by Reuters.

The timing of the hacking attempt this close to the November 3 elections, and lack of information from the FEC, stirred up conspiracy theories throughout the industry, several people said. White noted that it would be a good way for unscrupulous actors to hide unpopular last-minute spending.

"It's hard to say with any real information that that's what happened, but it's certainly I think where people's minds go very quickly," she said.

Kyle Sandy, the cybersecurity director at IT security firm Logically, said taking the system offline is the right call when dealing with an attack.

"It tells me the FEC detected something they couldn't immediately scope and chose a known outage over an unknown risk," Sandy said, giving the agency time to ensure none of its data was altered by the hackers before bringing it back online.