SAN JUAN, Puerto Rico (CNN) -- Search operations resumed Monday for six U.S. citizens missing since their small plane crashed Sunday evening off the north coast of Puerto Rico.

The U.S. Coast Guard found debris late Sunday night in the area where the single-engine Cessna 206 went down, half a nautical mile from the shoreline of the city of Quebradillas.

But despite five searches Sunday and two others throughout the night, rescue crews were not able to find the pilot or the five passengers from the plane, the Coast Guard said Monday.

Three searches are planned for Monday, said Coast Guard spokesman Ricardo Castrodad.

The private plane, chartered by Tropical Aviation Corp., took off from the Dominican Republic and was on its way to an airport in Puerto Rico when it went down Sunday evening, officials said.

Three male and two female passengers were returning to Puerto Rico after spending the weekend in the Dominican Republic, said Noemi Corporan, service manager for Tropical Aviation Corp. The passengers were San Juan residents and had flown to the Dominican Republic on Friday, she said.

Four HH-65 Dolphin helicopters from Coast Guard Air Station Borinquen and the Coast Guard Cutter Matinicus are taking part in the search, said spokesman Castrodad.

The airplane took off from Casa de Campo International Airport in the Dominican Republic and was supposed to land at the Luis Muñoz Marin International Airport in Carolina to clear U.S. Customs before going on to the Isla Grande Airport in San Juan, the Coast Guard said.

A 911 emergency operator notified the Coast Guard at 6:51 p.m. Sunday that an aircraft had crashed into the waters off Quebradillas.

The debris field was found around 10:30 p.m.

All About Puerto Rico • Dominican Republic • U.S. Coast Guard