D.C. voters had to wait four hours after polls closed Tuesday before receiving meaningful election returns after problems with a handful of voting machines led to an unusually lengthy and chaotic tabulation process.

The long wait — unusually long even by the generous standards of the D.C. Board of Elections — led to criticism from candidates and their supporters, left to stand around for hours waiting to hear the results of a closely-watched mayoral race among other contests.

Mayor Vincent C. Gray addressed the delay before conceding his re-election bid to D.C. Council member Muriel E. Bowser around midnight: “We’ve got some work to do there” at the Board of Elections, he said. “We probably have known that for a while. It seems to me [with the small] number of ballots that were cast, we should have been able to get a valid conclusion to this well before now.”

Questions about the late returns started arising about 9:30 p.m., 90 minutes after polls closed. In other recent elections, officials had posted the results of early voting not long after polling ended on Election Day. But on Tuesday, early voting totals were not released until nearly 10 p.m., and even then results were available for only 83 of the city’s 143 precincts.

But the real confusion ensued about a half-hour later, when the first Election Day results were released. While printouts provided at the board’s headquarters and a readout on the board’s web site indicated that 55 of 143 precincts were reporting, only about 17,000 votes were reported in an election that was expected to attract 100,000 voters or more. Later, results for 116 precincts were reported, still accounting for only about 45,000 votes.

Tamara Robinson, a board spokeswoman, said that only paper ballots had been tallied due to issues with tabulating results from the touch-screen electronic voting machines also deployed alongside the paper ballot scanners at every precinct. She did not initially say which precincts were being reported or which precincts’ electronic voting machines were problematic.

This year, for the first time, every precinct had at least two electronic machines, Robinson said, and many precinct workers did not know how to properly shut down multiple machines, leading to the long delays. Machines in five precincts were responsible, she said: Precinct 1, at Walker-Jones Education Center in Ward 6; Precinct 30, at Janney Elementary School in Ward 3; Precinct 36, at the Latin American Youth Center in Ward 1; Precinct 90, at Tyler Elementary School in Ward 6; and Precinct 140, at Anacostia High School in Ward 8.

Robinson said that vote-counters noticed inconsistent numbers reported in those precincts, so they stopped releasing tallies from the electronic machines until they could examine them more closely. They found that five or six of the machines had not been shut down correctly by poll workers, who may have been overwhelmed by the larger number of electronic machines at precincts this year.

At 11:38 p.m., the board released its first results from electronic machines, including tallies from 127 or 143 precincts. Shortly afterward, Gray conceded the mayoral race and Bowser accepted the Democratic nomination.

Clifford Tatum, the board’s executive director, said that the tallying took about an hour and ten minutes longer than he had hoped, but that on the whole, he was pleased with the expansion of the touch-screen machines this year.

Tatum said that there were more than twice as many of the machines this year, including at least two at every single precinct, which was taxing for poll workers. “Some of our workers have admittedly never touched laptops before,” he said.

The process of correctly shutting down one of the voting machines takes about ten to 15 minutes, Tatum said. Shutting down two machines is more complicated than simply doing the same process twice. Workers must transfer a memory cartridge from one machine to the next in order to print a paper audit tape that tallies both machines’ votes.

At the Board of Elections, counters noticed that five of the paper audit tapes were missing a serial number, meaning at least one machine’s votes in that precinct had not made it onto the electronic cartridge. So they drove off into the night to find those missing machines and download their votes.

“It took a little longer than what we’re used to,” Tatum acknowledged. “We are confident that the results are accurate, which is what we’re always concerned about — accuracy over speed.”

Tatum said the board will focus on training poll workers before the next election and may consider asking the council for more money to buy newer voting technology, which is easier to shut down without errors. During the day, he added, the board got many calls from citizens who wished there were more touch-screen machines at their polls.

Far more voters chose to cast paper ballots on Election Day: 43,440 picked paper, while 29,060 used the touch-screen machines. But the electronic machines were much more popular for early voters — 9,586 touch-screen ballots versus 954 on paper — because they were the only option offered at 12 of 13 early-voting sites.

Shortly before 12:30 a.m. Wednesday, Robinson said a complete tally of all precincts would be released within 10 minutes. Full results for all 143 precincts were finally reported at 1:40 a.m.

Aaron C. Davis contributed to this report.