“What’s clear so far is that the National Intelligence Service and other state agencies had engaged in a systematic and massive intervention in elections,” Kim Han-gil, the top opposition party leader, said on Thursday.

The intelligence service said its online messages were posted as part of normal psychological warfare operations against North Korea, which it said used the Internet to criticize South Korean government policies, forcing its agents to defend them online. In a statement on Thursday, it also accused the prosecutors of citing as their evidence online postings that had nothing to do with its agents.

The allegation first surfaced during the election campaign last year, when opposition politicians and officials from the National Election Commission tried in vain to enter an office in Seoul that was locked from the inside by an intelligence agent who refused to answer questions about whether she was part of an illegal online election effort.

Three days before the presidential election, the Seoul police announced that they had found no evidence to support the opposition’s accusations. During her last television debate, Ms. Park excoriated her main opposition rival, Moon Jae-in, over what she called the harassment of a female agent by his party.

But the scandal did not die with Ms. Park’s election.

A senior police investigator told reporters after the election that her supervisor had intervened in the investigation, withholding evidence. The boss, Kim Yang-pan, who is the former chief of the Seoul Metropolitan Police, was indicted with Mr. Won, the former intelligence chief. Both denied the charges against them.

At the time of Mr. Won’s indictment in June, the prosecutors said they had found thousands of online political postings by his agents since 2009. Then, last month, they said they had found more than 55,000 Twitter messages from them. The former head of the prosecutors’ investigation also said his boss in the Seoul Central District Prosecutors’ Office tried to block him from submitting that additional evidence, a charge the boss denied.

In a separate inquiry, military investigators are looking into South Korea’s Cyberwarfare Command after it was revealed last month in Parliament that some of its officials had conducted a similar online campaign against opposition candidates. The Cyberwarfare Command was created in 2010 to guard South Korea against hacking threats from North Korea.

On Thursday, the prosecutors said the 1.2 million Twitter messages they had discovered were mostly copies of the 26,500 original messages that the agents mass-distributed through a special computer program. But even if they were copies, they constituted an act of meddling in domestic politics and elections, Lee Jin-han, a senior prosecutor, told reporters.