Two men wanted in connection with the fatal shooting of a female officer in Bradford might be assisted in leaving the country, detectives fear.
Muzzaker Imtiaz Shah and Mustaf Jama may try to leave the UK
Officers have warned anyone helping Muzzaker Imtiaz Shah, 24, and Mustaf Jama, aged 25, faces prosecution.
On Friday a reward for information leading to convictions in the case was increased from £50,000 to £90,000.
Pc Sharon Beshenivsky died on 18 November. Yusuf Jama, 19, has been charged with her murder.
A clampdown is in place at airports, ports and ferry terminals as the search for Mr Shah and Mustaf Jama continues.
West Yorkshire Police revealed at a press conference the men have links to Sheffield, Birmingham and Leicester.
Mr Shah, who is orginally from Burnley, is known by the street name of 'Pesci' or 'P'. His last known address is in Kenton, London.
There is a strong possibility he may still be in London, detectives said.
Det Supt Andy Brennan who is leading the murder inquiry told a news conference: "I think that it is highly likely that there's a small group of people assisting them at this time.
"And I would urge any member of that group to contact the police as a matter of urgency.
Pc Beshenivsky was shot at a travel agency
"Likewise if you have suspicions about anybody who may be acting strangely, whether it's a neighbour or a friend, you have genuine concerns that they may be assisting either of these men, please contact us as a matter of urgency".
He said "intensive" inquiries are continuing in the search.
"All ports alerts were given out the moment we identified them as being of interest to the inquiry and their details were circulated to all police forces across the country," he said.
"It is highly probable that these men will try to leave the country. But in reality there is only a limited number of ways to do this."
The force has been liaising with customs and immigration officials in the inquiry.