BERLIN — Hundreds of police officers remained on alert in Munich on Friday after a threat of a suicide bombing attack by the Islamic State led the authorities to evacuate two train stations on New Year’s Eve.

The German authorities said Friday that five to seven people might have been involved in the terrorist threat. The two stations, in the city center and in the Pasing district in the western part of Munich, have reopened as the authorities continue to investigate.

Yet with little to go on and no arrests nearly 24 hours later, officials in Bavaria were defending their decision to close the two transit hubs hours before midnight and to flood the city with heavily armed, specially equipped officers — 550 as of Friday morning, including reinforcements from other parts of the southern state.

They said they had received a “very concrete tip” around 7:40 p.m. from intelligence sources in France and the United States indicating that militants from Iraq and Syria were planning to carry out attacks. By midday Friday, the immediate threat of an imminent attack had been lifted, although the police remained on general alert, said Joachim Herrmann, interior minister of Bavaria.