The Environmental Protection Agency is conducting what lawmakers are calling the most comprehensive study to date of the effects of a 1950 spill that has left a 55-acre plume of oil in and around Newtown Creek in Greenpoint, Brooklyn.

Federal investigators are examining the clean-up efforts åthat have been undertaken so far and will make recommendations to Congress by July as to what actions are needed to complete decontamination of the site, which sits under hundreds of homes and businesses, officials said yesterday. The study, which began in August, is being carried out with federal funding secured by Reps. Nydia Velazquez and Anthony Weiner, both Democrats of New York.

The Coast Guard, which in 1978 discovered at least 17 million gallons of oil underground in Greenpoint, has pegged the spill to an oil tank explosion at a former Standard Oil depot. The spill is believed to be the largest in the nation’s history, and it has prompted two separate lawsuits seeking billions of dollars in damages from big oil companies, principally Exxon Mobil, which agreed to clean up the site in a deal signed with the state in 1990.

Reports have emerged of multiple cases of bone cancer in the area that residents fear could be linked to the leak, and state and Exxon Mobil officials last month found elevated levels of potentially explosive and cancerous vapors.

“An aggressive study of the spill is needed,” Senator Schumer said yesterday as he stood next to the creek with Ms. Velazquez, Mr. Weiner, and other officials. “Answers are needed, once and for all.”

Mr. Schumer called for expanded testing for toxic vapors and for the creation of a local health registry for residents.

Messrs. Schumer and Weiner placed the blame squarely on Exxon Mobil, saying the company had tried to cover up the spill and had delayed in cleaning it up.”If Exxon Mobil has their way, this problem will be solved in 50 — fivezero — years” Mr. Weiner said. He added, “This is not something we can afford to wait much longer.”

Exxon Mobil is contesting the lawsuits and says it is complying with its agreement with the state.The company says it has cleaned up 9.3 million of the 17 million gallons of oil believed to be underground.A spokesman, Brian Dunphy, would say only that the rest of the cleanup will take “a number of additional years” to complete. “We do not believe there are any health or safety impacts from the underground petroleum plumes attributable to our former facilities,” Mr. Dunphy said in a company statement. Of the toxic vapors, he said they are likely from natural gas leaks, “not from Exxon Mobil’s historic operations.”

The lawmakers yesterday said the EPA study was expected to cost up to $2.5 million, which will be paid out of a liability trust fund that collects payments from the major oil companies under federal law.

Despite heightened concerns about the health effects of the spill, definitive answers may not come from the EPA study.An agency spokesman, Benjamin Barry, said investigators were focused on the cleanup efforts and not on direct links to illness.”The EPA is not a health agency,” he said.

While Democrats have criticized the Bush administration for its ties to major oil companies, the party’s legislators expressed confidence that the EPA would put out an objective report. “We’ll be keeping an eye on things so that there’s not political pressure and the truth can come out,” Mr. Schumer said.